fix(transforms): convert view_copy to unsqueeze when the added dim is 0

[durvesh1992]

Summary

ViewCopyToSqueezeUnsqueezePass rewrites a view_copy that adds a size-1 dimension into an unsqueeze_copy. It decides whether to do the rewrite with:

unsqueeze_dim = self.find_unsqueeze_dim(input_shape, view_shape)
if unsqueeze_dim:
    ...

find_unsqueeze_dim returns the index of the added size-1 dimension, and None when the shapes don't match an unsqueeze. When the size-1 dim is added at the front, the returned index is 0 — which is falsy — so if unsqueeze_dim: skips the rewrite and the view_copy is left in place for that case (e.g. [3, 4] -> [1, 3, 4]).

The fix is to guard on is not None:

if unsqueeze_dim is not None:

(The squeeze branch is unaffected: find_squeeze_dims returns a non-empty list or None, so its if squeeze_dims: check is correct.)

Test plan

Adds backends/transforms/test/test_view_copy_to_squeeze_unsqueeze.py with three cases: unsqueeze at dim 0 (regression for this bug), unsqueeze at dim 1, and squeeze.

• Before the fix, test_unsqueeze_at_dim_0 fails — the view_copy is not converted (graph still contains aten.view_copy).
• After the fix, all three tests pass.

Run:

python -m unittest backends.transforms.test.test_view_copy_to_squeeze_unsqueeze -v

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