Fix aten.copy: wrap CastLike with Identity to prevent IR value aliasing

[Copilot]

torch.onnx.export raised ValueError when exporting an ExportedProgram containing register_buffer + in-place copy (copy_), because aten_copy's op.CastLike(src, self) returns the same IR value object as src when both tensors share the same dtype — no new ONNX node is created. A subsequent unconditional rename of that value clobbered the original buffer placeholder name (e.g. b_prompt_feat → copy), causing initializer registration to fail.

Change

• onnxscript/function_libs/torch_lib/ops/core.py: Wrap the CastLike result in op.Identity() so aten_copy always produces a fresh IR value, leaving the source value's name intact.

# Before
def aten_copy(self: TTensor, src: TTensor2, non_blocking: bool = False) -> TTensor:
    return op.CastLike(src, self)

# After
def aten_copy(self: TTensor, src: TTensor2, non_blocking: bool = False) -> TTensor:
    return op.Identity(op.CastLike(src, self))

This matches the existing pattern in aten__to_copy, which already returns op.Identity(self) for the same-dtype case.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 71.96%. Comparing base (1077da7) to head (83ee953).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
onnxscript/function_libs/torch_lib/ops/core.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2875   +/-   ##
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  Coverage   71.96%   71.96%           
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  Files         239      239           
  Lines       29224    29224           
  Branches     2878     2878           
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  Hits        21031    21031           
  Misses       7216     7216           
  Partials      977      977           

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[justinchuby]

Actually we should probably fix the castlike evaluation logic to emit an identity also.