Update type of create_block_mask to more accurately reflect things

[Chillee]

Fixes some mypy issues

[pytorch-bot]

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✅ You can merge normally! (3 Unrelated Failures)

As of commit 929bb86 with merge base 65139eb ():

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• trunk / macos-py3-arm64-mps / test (mps, 1, 1, macos-m1-13) (gh) (matched macos rule in flaky-rules.json)
  File doesn't exist

BROKEN TRUNK - The following job failed but was present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / linux-jammy-py3-clang12-executorch / build (gh) (trunk failure)
  Final attempt failed. Child_process exited with error code 1

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / cuda12.4-py3.10-gcc9-sm75 / test (pr_time_benchmarks, 1, 1, linux.g4dn.metal.nvidia.gpu) (gh) (#149370)
  REGRESSION: benchmark ('basic_modules_ListOfLinears_inductor_gpu_force_shape_pad', 'compile_time_instruction_count') failed, actual result 15651381449 is 1.57% higher than expected 15410000000 ±+1.50% if this is an expected regression, please update the expected results.

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[drisspg]

Why the runtime import OOC

[Chillee]

@drisspg It causes flex_attention to import dynamo which then imports fsdp and adds a bunch of import time.

[Chillee]

@pytorchbot merge -i

[pytorchmergebot]

Merge started

Your change will be merged while ignoring the following 2 checks: pull / linux-jammy-py3-clang12-executorch / build, pull / cuda12.4-py3.10-gcc9-sm75 / test (pr_time_benchmarks, 1, 1, linux.g4dn.metal.nvidia.gpu)

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