Move BlockSparseWeightConfig to prototype and deprecate torchao.sparsity imports (#4230)

[jerryzh168]

Stack from ghstack (oldest at bottom):

• -> Move BlockSparseWeightConfig to prototype and deprecate torchao.sparsity imports (#4230) #4232

BlockSparseWeightConfig, BlockSparseTensor, and related code have been
moved from torchao.sparsity to torchao.prototype.sparsity. Stub imports
remain in the old location and emit DeprecationWarnings to guide users
to the new paths. The block sparse test has also been moved to
test/prototype/test_blocksparse.py.

Deprecation Note:

Importing BlockSparseWeightConfig (or its alias block_sparse_weight)
from torchao.sparsity is deprecated. Please update your imports:

# Before (deprecated):
from torchao.sparsity import block_sparse_weight
from torchao.sparsity import BlockSparseWeightConfig
from torchao.sparsity.blocksparse import BlockSparseTensor

# After:
from torchao.prototype.sparsity import block_sparse_weight
from torchao.prototype.sparsity import BlockSparseWeightConfig
from torchao.prototype.sparsity.blocksparse import BlockSparseTensor

The old import paths will be removed in a future release.
See #4230 for more details.

[pytorch-bot]

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❌ 6 New Failures, 2 Unrelated Failures

As of commit 3a29570 with merge base a302c10 ():

NEW FAILURES - The following jobs have failed:

• Run Regression Tests / test (CPU 2.8, linux.4xlarge, torch==2.8.0 torchvision==0.23.0 --index-url https://download.pytor... / linux-job (gh)
  RuntimeError: Command docker exec -t 51ac81cdabbb5549e19738fffe9b7fe8e086015aa5f9565bc712233ba705665f /exec failed with exit code 2
• Run Regression Tests / test (CUDA 2.10, linux.g5.12xlarge.nvidia.gpu, torch==2.10.0 torchvision==0.25.0, cuda, 12.6) / linux-job (gh)
  RuntimeError: Command docker exec -t f7e57a99974567e2727c49195e9fabca798e0c20f1c0d8bac2947e0cbeffe1f8 /exec failed with exit code 2
• Run Regression Tests / test (CUDA 2.8, linux.g5.12xlarge.nvidia.gpu, torch==2.8.0 torchvision==0.23.0, cuda, 12.6) / linux-job (gh)
  RuntimeError: Command docker exec -t 4dfb38bfb0f7d87cb15cb0f35271a856708ca3d869a60082c02be62ae5321868 /exec failed with exit code 2
• Run Regression Tests / test (CUDA 2.9, linux.g5.12xlarge.nvidia.gpu, torch==2.9.1 torchvision==0.24.1, cuda, 12.6) / linux-job (gh)
  RuntimeError: Command docker exec -t c78ffe5a39b30ee127f0fc4b8d5e8a575d2389f596a88b81447ea43c14af80ea /exec failed with exit code 2
• Run Regression Tests / test-nightly (CPU Nightly, linux.4xlarge, --pre torch torchvision --index-url https://download.py... / linux-job (gh)
  RuntimeError: Command docker exec -t 70ad152d1374f29f75e19f8685668630a2d52ef39ea64debca085bf727d45972 /exec failed with exit code 2
• Run Regression Tests / test-nightly (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch torchvision --index-url htt... / linux-job (gh)
  RuntimeError: Command docker exec -t a057bd1a26a0e9e5444e4107d0b8da611e24a0880b636a2485443da032be39fe /exec failed with exit code 2

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• Run Regression Tests / test (CPU 2.10, linux.4xlarge, torch==2.10.0 torchvision==0.25.0 --index-url https://download.pyt... / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• Run Regression Tests / test (CPU 2.9, linux.4xlarge, torch==2.9.1 torchvision==0.24.1 --index-url https://download.pytor... / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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