elementwise_util: don't cast the result of compute_fun back to the common type

[swolchok]

The compute function might return an entirely different type. For
example, if we were applying a trigonometric function like acos to an
input of type bool expecting an output of type float, we would get bad
results because acos(0) = 1.57, but casting through bool would
truncate that to 1.

Note that we don't need the pair of ET_CHECK_MSG I removed because we
already check tensor dtypes on entry to the elementwise util
functions; the checks were inconvenient because we now call
get_store_common_to_tensor_fn without the actual common type.

[swolchok]

Stack from ghstack (oldest at bottom):

• RFC: Specialize for non-mixed-dtype in elementwise_util #9388
• Refactor elementwise_util: create variants with out_dtypes in template argument list #9387
• Implement unary_ufunc functions using elementwise_util #9386
• -> elementwise_util: don't cast the result of compute_fun back to the common type #9385
• Make PyTorch headers available in optimized_portable_kernels, define ET_USE_PYTORCH_HEADERS #9384

[pytorch-bot]

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❌ 7 New Failures

As of commit 9fcd885 with merge base b195ed9 ():

NEW FAILURES - The following jobs have failed:

• pull / test-binary-size-linux / linux-job (gh)
  /pytorch/executorch/../executorch/kernels/portable/cpu/util/dtype_util.h:176:18: error: unused variable 'common_scalar_type' [-Werror,-Wunused-variable]
• pull / test-binary-size-linux-gcc / linux-job (gh)
  /pytorch/executorch/../executorch/kernels/portable/cpu/util/dtype_util.h:176:18: error: unused variable ‘common_scalar_type’ [-Werror=unused-variable]
• pull / unittest / linux / linux-job (gh)
  [ FAILED ] OpRSubScalarOutTest.ShortTensors
• pull / unittest / macos / macos-job (gh)
  [ FAILED ] OpRSubScalarOutTest.ShortTensors
• pull / unittest-buck / macos / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3
• pull / unittest-editable / linux / linux-job (gh)
  [ FAILED ] OpRSubScalarOutTest.ShortTensors
• pull / unittest-editable / macos / macos-job (gh)
  [ FAILED ] OpRSubScalarOutTest.ShortTensors

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