[mlir][tensor] Introduce TensorRelayoutOpInterface (llvm#125823)

The newly introduced `TensorRelayoutOpInterface` is created specifically
for `tensor.pack` + `tensor.unpack`. Although the interface is
currently empty, it enables us to refactor the logic in
`FoldTensorCastProducerOp` within the Tensor dialect as follows:

```cpp
// OLD
// Reject tensor::PackOp - there's dedicated pattern for that instead.
if (!foldTensorCastPrecondition(op) ||
    isa<tensor::PackOp, tensor::UnPackOp>(*op))
  return failure();
```

is replaced with:

```cpp
// NEW
// Reject tensor::PackOp - there's dedicated pattern for that instead.
if (!foldTensorCastPrecondition(op) ||
    isa<tensor::RelayoutOpInterface>(*op))
  return failure();
```

This will be crucial once `tensor.pack` + `tensor.pack` are replaced
with `linalg.pack` + `linalg.unpack` (i.e. moved to Linalg):
  * llvm#123902,
  * https://discourse.llvm.org/t/rfc-move-tensor-pack-and-tensor-unpack-into-linalg/.

Note that the interface itself will later be moved to the Linalg
dialect. This decoupling ensures that the Tensor dialect does not
require an understanding of Linalg ops, thus keeping the dependency
lightweight.

This PR is effectively a preparatory step for moving PackOp and UnpackOp
to Linalg. Once that's completed, most CMake changes from this PR will
be effectively reverted.

mlir/include/mlir/Dialect/Tensor/IR/CMakeLists.txt

@@ -1,2 +1,8 @@
 add_mlir_dialect(TensorOps tensor)
 add_mlir_doc(TensorOps TensorOps Dialects/ -gen-dialect-doc)
+
+set(LLVM_TARGET_DEFINITIONS TensorInterfaces.td)
+mlir_tablegen(TensorInterfaces.h.inc -gen-op-interface-decls)
+mlir_tablegen(TensorInterfaces.cpp.inc -gen-op-interface-defs)
+add_public_tablegen_target(MLIRTensorInterfacesIncGen)
+add_dependencies(mlir-headers MLIRTensorInterfacesIncGen)

mlir/include/mlir/Dialect/Tensor/IR/Tensor.h

@@ -46,6 +46,12 @@ SmallVector<Range, 8> getOrCreateRanges(OffsetSizeAndStrideOpInterface op,
 
 #include "mlir/Dialect/Tensor/IR/TensorOpsDialect.h.inc"
 
+//===----------------------------------------------------------------------===//
+// Tensor Interfaces
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Dialect/Tensor/IR/TensorInterfaces.h.inc"
+
 //===----------------------------------------------------------------------===//
 // Tensor Dialect Operations
 //===----------------------------------------------------------------------===//

mlir/include/mlir/Dialect/Tensor/IR/TensorInterfaces.td

@@ -0,0 +1,33 @@
+//===- TensorInterfaces.td - Tensor Interfaces Declaration -*- tablegen -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This is the definition file for the structured interface sfor Tensor ops.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef TENSOR_IR_TENSORINTERFACES
+#define TENSOR_IR_TENSORINTERFACES
+
+include "mlir/Interfaces/DestinationStyleOpInterface.td"
+include "mlir/IR/OpBase.td"
+
+// TODO: To be moved to LinalgInterfaces.td, see:
+//  * https://github.com/llvm/llvm-project/pull/123902
+//  * https://discourse.llvm.org/t/rfc-move-tensor-pack-and-tensor-unpack-into-linalg/
+def TensorRelayoutOpInterface : OpInterface<"RelayoutOpInterface"> {
+  let description = [{
+    A Tensor (soon to be Linalg) relayout-op is either tensor.pack or
+    tensor.unpack.
+
+    While we could extend this interface with methods from Tensor_RelayoutOp,
+    this is currently not needed and left as a TODO.
+  }];
+  let cppNamespace = "::mlir::tensor";
+}
+
+#endif // TENSOR_IR_TENSORINTERFACES

mlir/include/mlir/Dialect/Tensor/IR/TensorOps.td

@@ -10,6 +10,7 @@
 #define TENSOR_OPS
 
 include "mlir/Dialect/Tensor/IR/TensorBase.td"
+include "mlir/Dialect/Tensor/IR/TensorInterfaces.td"
 include "mlir/Interfaces/CastInterfaces.td"
 include "mlir/Interfaces/ControlFlowInterfaces.td"
 include "mlir/Interfaces/DestinationStyleOpInterface.td"
@@ -1833,6 +1834,7 @@ class Tensor_RelayoutOp<string mnemonic, list<Trait> traits = []> :
         DestinationStyleOpInterface,
         ConditionallySpeculatable, NoMemoryEffect,
         DeclareOpInterfaceMethods<ReifyRankedShapedTypeOpInterface>,
+        TensorRelayoutOpInterface,
         TypesMatchWith<"result type matches type of dest",
                    "dest", "result",
                    "$_self">])> {

mlir/lib/Dialect/Tensor/IR/CMakeLists.txt

@@ -16,6 +16,7 @@ add_mlir_dialect_library(MLIRTensorDialect
 
   DEPENDS
   MLIRTensorOpsIncGen
+  MLIRTensorInterfacesIncGen
 
   LINK_LIBS PUBLIC
   MLIRAffineDialect

mlir/lib/Dialect/Tensor/IR/TensorOps.cpp

@@ -4976,7 +4976,7 @@ struct FoldTensorCastProducerOp
 
     // Reject tensor::PackOp - there's dedicated pattern for that instead.
     if (!foldTensorCastPrecondition(op) ||
-        isa<tensor::PackOp, tensor::UnPackOp>(*op))
+        isa<tensor::RelayoutOpInterface>(*op))
       return failure();
 
     SmallVector<Type> newResultTypes(op->getResultTypes());