Remove over-strict softmax mask divisibility assert

[ethansfng]

Summary: SoftmaxPattern.fuse() asserts mask_shape[-1] % 16 == 0. The softmax mask passed to the fused op is a dummy (mask_type=0, no masking is applied), so its trailing dimension does not affect numerics, and the historical QuantFusion simply floor-divided without asserting. The assert rejects otherwise-valid shapes (e.g. softmax over a last dim of 17 or 33) and fails test_quantized_softmax_out_* (T273477740). Remove the assert and floor-divide the mask shape like before, in both the fbcode/ and xplat/ cells.

Differential Revision: D106957459

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