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.

[gramalingam]

@copilot: op.CastLike is already an operator that creates a new value (just like op.Identity). So, this fix doesn't sound like the right fix. I suspect the problem may be caused by an optimization (or rewrite-rule) that optimizes CastLike when it is redundant. If so, the right fix may be in that optimization (introducing an Identity under special circumstances). It is worth digging more to into the optimization of CastLike (either under the optimizer or rewriter folder).