Add a small fastpath test for native mha (#81432)
Summary: We dont have a small fast path passing test for mha before, this diff added one for better testing
Test Plan: buck build mode/dev-nosan -c fbcode.platform=platform009 -c fbcode.enable_gpu_sections=true caffe2/test:nn && buck-out/dev/gen/caffe2/test/nn\#binary.par -r test_multihead_attn_fast_path_small_test
Differential Revision: D37834319
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81432
Approved by: https://github.com/erichan1
