Fix contracting dim match and batch broadcasting in broadcast_to_matmul

[Hardik K (hardikkaurani)]

Summary

In _broadcast_to_matmul.py, check_if_not_need_reshape inspected reversed input dimensions with:

if dim_from_a not in {1, dim_from_b}:
    return False

This introduced two issues:

1. Invalid graph rewrite on contracting dimension mismatch (idx == 0):
   At idx == 0, dim_from_a and dim_from_b are the contracting dimensions $K$ of $A$ and $B$. In MatMul, $K$ cannot broadcast and must match exactly. Accepting dim_from_a == 1 when dim_from_b != 1 erroneously permitted rewriting valid reshape graphs into invalid MatMul nodes with mismatched contracting dimensions that fail ONNX shape inference.
2. Rejected rewrite for valid batch broadcasting (idx > 0):
   At idx > 0, dim_from_a and dim_from_b are batch dimensions. Two dimensions are broadcastable if $d_A = d_B$, $d_A = 1$, or $d_B = 1$. The previous check rejected valid broadcasting when $d_B = 1$ and $d_A > 1$ (e.g. $[2, 3, 4]$ and $[1, 4, 5]$).

Changes

• Enforce exact equality on contracting dimensions at idx == 0 (dim_from_a == dim_from_b).
• Enable standard symmetric broadcasting on batch dimensions at idx > 0 (dim_from_a == dim_from_b or dim_from_a == 1 or dim_from_b == 1).
• Added regression unit tests to _broadcast_to_matmul_test.py:
  • test_reshape_matmul_reshape_replace_when_second_input_batch_dim_is_one: confirms redundant reshapes are removed when input $B$ has batch dimension 1.
  • test_reshape_matmul_reshape_remain_when_contracting_dimension_mismatches: confirms the rewrite is safely declined when contracting dimensions mismatch.

Validation

• All 18 unit tests in _broadcast_to_matmul_test.py pass.
• All 322 common rewriter rules tests pass (1 existing skip).
• ruff check and ruff format --check pass with zero warnings or errors.

[Hardik K (hardikkaurani)]

Hello G. Ramalingam (@gramalingam),

I have submitted this pull request to resolve two correctness issues in _broadcast_to_matmul.py:

1. When contracting dimensions mismatch at idx == 0, exact equality is now required rather than allowing dim_from_a == 1, preventing invalid rewrites into incompatible MatMul operations.
2. When batch dimensions broadcast symmetrically at idx > 0, the check now correctly accounts for dim_from_b == 1 as well as dim_from_a == 1.

All 18 unit tests in _broadcast_to_matmul_test.py and all 322 common rewrite tests pass, and ruff checks and formatting are clean.

Please review when convenient and let me know if any adjustments are desired.