Allow users to use the customized model

**What does this PR do?**
1. This PR introduce ModelSpec to decribe a model and how to parallelize a model.
2. All the models should define `build_model_spec()` or `model_spec` to
   be imported by the `model` module.
3. `build_model_specs()` is called in the trainer to get the `model_specs` and the result is used to get the corresponding model spec.
4. Users can also use `--experimental.model_module_path` to dynamically import a model that is not implemented by TorchTitan.

**Why do we need this PR?**
This allows users to use TorchTitan with a new model without intrusively change TorchTitan code.

**Next steps**
1. This PR only include the mode definitions, configurations, totkenizer, parallize_fn, and
   pipelining_fn.  We may also want to extend ModelSpec to include optimizer and lr_scheduler
2. Current TorchTitan parallelize and pipelining_fn import ModelArgs which can cause circular imports.
   We should fix this issue.

**What does this PR do?**
1. Introduces `ModelSpec` to describe a model and how to parallelize it.
2. Requires all models to define `build_model_spec()` or `model_spec`, which will be imported by the model module.
3. Calls `build_model_specs()` in the trainer to obtain `model_specs`, which are then used to retrieve the corresponding model spec.
4. Allows users to dynamically import a model not implemented by TorchTitan using --experimental.model_module_path.

**Why do we need this PR?**
This PR enables users to integrate new models with TorchTitan without making intrusive changes to the TorchTitan codebase.

**Next steps**
1. This PR includes only the model definitions, configurations, tokenizer, parallelize_fn, and pipelining_fn. We may want to extend ModelSpec to include the optimizer and learning rate scheduler.
2. The current TorchTitan parallelize and pipelining_fn import ModelArgs, which can lead to circular imports. This issue needs to be addressed.

ghstack-source-id: a88ff3ebe5c869055dd3314fb1b791855fd0e0b2
Pull Request resolved: #814

torchtitan/config_manager.py

@@ -375,6 +375,20 @@ def __init__(self):
                 The default value is 'allgather'.
             """,
         )
+        self.parser.add_argument(
+            "--experimental.custom_model_path",
+            type=str,
+            default="",
+            help="""
+                The --custom_model_path option allows to specify a custom path to a model module
+
+                that is not natively implemented within TorchTitan.
+
+                Acceptable values are the file system path to the module (e.g., my_models/model_x)
+
+                dotted import module  (e.g., some_package.model_x).
+            """,
+        )
         self.parser.add_argument(
             "--training.mixed_precision_param",
             type=str,
@@ -638,6 +652,15 @@ def parse_args(self, args_list: list = sys.argv[1:]):
                 exp["pipeline_parallel_split_points"]
             )
 
+        if (
+            "experimental" in args_dict
+            and "model_module_path" in args_dict["experimental"]
+            and args_dict["experimental"]["model_module_path"]
+        ):
+            from torchtitan.models import add_model_spec_path
+
+            add_model_spec_path(args_dict["experimental"]["model_module_path"])
+
         # override args dict with cmd_args
         cmd_args_dict = self._args_to_two_level_dict(cmd_args)
         for section, section_args in cmd_args_dict.items():

torchtitan/model_spec.py

@@ -0,0 +1,28 @@
+from dataclasses import dataclass
+from typing import Callable, Dict, List, Protocol, Tuple, Type
+
+import torch.nn as nn
+from torch.distributed.pipelining.schedules import _PipelineSchedule
+
+
+@dataclass
+class BaseModelArgs:
+    _enforced: str = "This field is used to enforce all fields have defaults."
+
+
+class ModelProtocol(Protocol):
+    def from_model_args(self, args: BaseModelArgs) -> nn.Module:
+        ...
+
+
+@dataclass
+class ModelSpec:
+    name: str
+    cls: Type[nn.Module]
+    config: Dict[str, BaseModelArgs]
+    # As for now, this is a string. So it will have to be built-in to the
+    # TorchTitan library. In the future, we can make this a defined class
+    # that can be extended like ModelSpec.
+    tokenizer: str
+    parallelize_fn: Callable[[nn.Module], None]
+    pipelining_fn: Callable[[nn.Module], Tuple[_PipelineSchedule, List[nn.Module]]]

torchtitan/models/__init__.py

@@ -4,14 +4,85 @@
 # This source code is licensed under the BSD-style license found in the
 # LICENSE file in the root directory of this source tree.
 
-from torchtitan.models.llama import llama3_configs, Transformer
+import importlib
 
-models_config = {
-    "llama3": llama3_configs,
-}
+import os
+import pkgutil
+from typing import Dict, Set
 
-model_name_to_cls = {"llama3": Transformer}
+import torchtitan.models as models
+from torchtitan.model_spec import ModelSpec
 
-model_name_to_tokenizer = {
-    "llama3": "tiktoken",
-}
+
+_model_specs_path: Set[str] = set()
+
+
+def _load_module(path: str):
+    path = os.path.expanduser(path)
+
+    # 1. Check if path is an existing file or directory path.
+    if os.path.exists(path):
+        if os.path.isdir(path):
+            init_file = os.path.join(path, "__init__.py")
+            if os.path.isfile(init_file):
+                return _load_module_from_init(path)
+
+            raise ImportError(
+                f"Directory '{path}' is not a Python package because it does not "
+                "contain an __init__.py file."
+            )
+        else:
+            raise ImportError(f"Path '{path}' is not a directory.")
+
+    # 2. If not a valid path, assume it's a dotted module name.
+    return importlib.import_module(path)
+
+
+def _load_module_from_init(path: str):
+    module_name = os.path.basename(os.path.normpath(path))
+    init_file = os.path.join(path, "__init__.py")
+
+    spec = importlib.util.spec_from_file_location(module_name, init_file)
+    if spec is None:
+        raise ImportError(f"Could not create spec from '{init_file}'")
+
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
+for _, name, _ in pkgutil.iter_modules(models.__path__):
+    full_module_name = f"{models.__name__}.{name}"
+    _model_specs_path.add(full_module_name)
+    # model_module = importlib.import_module(full_module_name)
+    # load_spec_from_module(model_module)
+
+
+def add_model_spec_path(path: str):
+    global _model_specs_path
+    _model_specs_path.add(path)
+
+
+def build_model_specs() -> Dict[str, ModelSpec]:
+    """
+    Load all model specs from the `models` package.
+    """
+    global _model_specs_path
+    model_specs = {}
+    for path in _model_specs_path:
+        module = _load_module(path)
+        model_spec = getattr(module, "model_spec", None)
+        if model_spec is not None:
+            model_specs[model_spec.name] = model_spec
+        # We would like to just use `model_spec` but current torchtitan parallelize
+        # functions depend on ModelArgs and can cause circular imports.
+        # As a result, we have to use `build_model_spec` as a workaround.
+        build_model_spec = getattr(module, "build_model_spec", None)
+        if build_model_spec:
+            model_spec = build_model_spec()
+            model_specs[model_spec.name] = model_spec
+
+    return model_specs
+
+
+__all__ = [add_model_spec_path, build_model_specs]

torchtitan/models/llama/__init__.py

@@ -6,9 +6,9 @@
 #
 # Copyright (c) Meta Platforms, Inc. All Rights Reserved.
 
+from torchtitan.model_spec import ModelSpec
 from torchtitan.models.llama.model import ModelArgs, Transformer
 
-__all__ = ["Transformer"]
 
 llama3_configs = {
     "debugmodel": ModelArgs(dim=256, n_layers=8, n_heads=16, rope_theta=500000),
@@ -40,3 +40,18 @@
         rope_theta=500000,
     ),
 }
+
+
+def build_model_spec() -> ModelSpec:
+    # Avoid circular import
+    from torchtitan.parallelisms.parallelize_llama import parallelize_llama
+    from torchtitan.parallelisms.pipeline_llama import pipeline_llama
+
+    return ModelSpec(
+        name="llama3",
+        cls=Transformer,
+        config=llama3_configs,
+        tokenizer="tiktoken",
+        parallelize_fn=parallelize_llama,
+        pipelining_fn=pipeline_llama,
+    )

torchtitan/models/llama/model.py

@@ -13,11 +13,12 @@
 import torch
 import torch.nn.functional as F
 from torch import nn
+from torchtitan.model_spec import BaseModelArgs, ModelProtocol
 from torchtitan.models.norms import build_norm
 
 
 @dataclass
-class ModelArgs:
+class ModelArgs(BaseModelArgs):
     dim: int = 4096
     n_layers: int = 32
     n_heads: int = 32
@@ -258,7 +259,7 @@ def init_weights(self, init_std: float):
             nn.init.trunc_normal_(linear.weight, mean=0.0, std=init_std)
 
 
-class TransformerBlock(nn.Module):
+class TransformerBlock(nn.Module, ModelProtocol):
     """
     TransformerBlock Module
 

train.py

@@ -19,13 +19,9 @@
 from torchtitan.float8 import Float8Handler
 from torchtitan.logging import init_logger, logger
 from torchtitan.metrics import build_device_memory_monitor, build_metric_logger
-from torchtitan.models import model_name_to_cls, model_name_to_tokenizer, models_config
+from torchtitan.models import build_model_specs
 from torchtitan.optimizer import build_lr_schedulers, build_optimizers
-from torchtitan.parallelisms import (
-    models_parallelize_fns,
-    models_pipelining_fns,
-    ParallelDims,
-)
+from torchtitan.parallelisms import ParallelDims
 from torchtitan.profiling import maybe_enable_memory_snapshot, maybe_enable_profiling
 from torchtitan.utils import device_module, device_type
 
@@ -80,9 +76,10 @@ def main(job_config: JobConfig):
         world_mesh, device, job_config.training.seed, job_config.training.deterministic
     )
     model_name = job_config.model.name
+    model_spec = build_model_specs()[model_name]
 
     # build tokenizer
-    tokenizer_type = model_name_to_tokenizer[model_name]
+    tokenizer_type = model_spec.tokenizer
     tokenizer = build_tokenizer(tokenizer_type, job_config.model.tokenizer_path)
     # build dataloader
     data_loader = build_hf_data_loader(
@@ -96,8 +93,8 @@ def main(job_config: JobConfig):
     )
 
     # build model (using meta init)
-    model_cls = model_name_to_cls[model_name]
-    model_config = models_config[model_name][job_config.model.flavor]
+    model_cls = model_spec.cls
+    model_config = model_spec.config[job_config.model.flavor]
     # set the model configs from training inputs:
     # 1. norm type to decide which norm layer to use
     # 2. vocab size from tokenizer
@@ -151,7 +148,7 @@ def loss_fn(pred, labels):
     # apply parallelisms and initialization
     if parallel_dims.pp_enabled:
         # apply PT-D Pipeline Parallel
-        pp_schedule, model_parts = models_pipelining_fns[model_name](
+        pp_schedule, model_parts = model_spec.pipelining_fn(
             model, pp_mesh, parallel_dims, job_config, device, model_config, loss_fn
         )
         # when PP is enabled, `model` obj is no longer used after this point, model_parts is used instead
@@ -162,14 +159,14 @@ def loss_fn(pred, labels):
         # optimizer, and checkpointing
         for m in model_parts:
             # apply SPMD-style PT-D techniques
-            models_parallelize_fns[model_name](m, world_mesh, parallel_dims, job_config)
+            model_spec.parallelize_fn(m, world_mesh, parallel_dims, job_config)
             m.to_empty(device=init_device)
             with torch.no_grad():
                 m.init_weights(buffer_device=buffer_device)
             m.train()
     else:
         # apply PT-D Tensor Parallel, activation checkpointing, torch.compile, Data Parallel
-        models_parallelize_fns[model_name](model, world_mesh, parallel_dims, job_config)
+        model_spec.parallelize_fn(model, world_mesh, parallel_dims, job_config)
         model.to_empty(device=init_device)
         with torch.no_grad():
             model.init_weights(buffer_device=buffer_device)