fix: prevent MXFP8 amax aliasing with dSoftmaxOffset in bwd

[vedaanta]

Summary

• In fused_attn_fp8_bwd, amax pointers for dQ/dK/dV all pointed to output_dQ->amax.dptr. For MXFP8 (which doesn't use per-tensor
  amax), PyTorch's caching allocator could reuse this memory for the dSoftmaxOffset tensor.
• cuDNN's second bprop kernel wrote amax_dQ to that address, corrupting d_softmax_offset[0].
• Fix: use each tensor's own amax/scale pointers, and when aliasing with dSoftmaxOffset is detected, allocate scratch space for the
  amax outputs.

Test plan

• test_dpa_fp8_vs_f16[mxfp8-True-True-bshd_bshd_bshd-fp8_16-dtype0] _ d_softmax_offset RMSE dropped from 0.60 to 0.15

[vedaanta]