sampled_addmm for BSR inputs: fuse softmax

[nikitaved]

Stack from ghstack (oldest at bottom):

• -> sampled_addmm for BSR inputs: fuse softmax #101978
• sampled_addmm backward: fix incorrect gradient wrt self #103548
• SDPA: frontend for BSR masks #104042

[pytorch-bot]

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 560ba85:

NEW FAILURE - The following job has failed:

• lintrunner / linux-job (gh)

UNSTABLE - The following job failed but was likely due to flakiness present on trunk and has been marked as unstable:

• linux-bionic-cuda12.1-py3.10-gcc9-bazel-test / build-and-test (default, 1, 1, linux.4xlarge.nvidia.gpu, unstable) (gh)

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