Fix aten_copy to wrap CastLike with Identity to prevent IR value aliasing

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Co-authored-by: justinchuby <11205048+justinchuby@users.noreply.github.com>

Author: Copilot

onnxscript/function_libs/torch_lib/ops/core.py

@@ -2515,7 +2515,7 @@ def aten_copy(
 ) -> TTensor:
     """copy(Tensor self, Tensor src, bool non_blocking=False) -> Tensor"""
 
-    return op.CastLike(src, self)
+    return op.Identity(op.CastLike(src, self))
 
 
 @torch_op("aten::_to_copy", trace_only=True)