1/x mx cleanup: mxtensor: rename _block_size to block_size

[vkuzo]

Summary:

Renames MXTensor._block_size to MXTensor.block_size to match other
tensors in AO.

Test Plan:

pytest test/prototype/mx_formats -s -x

Reviewers:

Subscribers:

Tasks:

Tags:

[vkuzo]

Stack from ghstack (oldest at bottom):

• 6/x mx cleanup: make NVFP4Tensor use base implements #3387
• 5/x mx cleanup: rename mx inf config to MXDynamicActivationMXWeightConfig #3386
• 4/x: mx cleanup: use kernel_preference instead of gemm_kernel_choice #3385
• 3/x mx cleanup: speed up local test suite #3383
• 2/x mx cleanup: remove pack_fp6 #3382
• -> 1/x mx cleanup: mxtensor: rename _block_size to block_size #3381

[pytorch-bot]

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❌ 3 New Failures

As of commit 0a18f34 with merge base 5f33595 ():

NEW FAILURES - The following jobs have failed:

• Run 1xH100 Tests / test (H100, linux.aws.h100, --pre torch torchvision torchaudio fbgemm-gpu-genai --index-url https... / linux-job (gh)
  RuntimeError: Command docker exec -t 02e620d8f189bc39e4c26547fe67e3ba53c0be20c4155f8bc79709c204aaa301 /exec failed with exit code 1
• Run 1xL4 Tests / test (SM-89, linux.g6.4xlarge.experimental.nvidia.gpu, --pre torch --index-url https://download.p... / linux-job (gh)
  RuntimeError: Command docker exec -t f5c2722cdb2ed7de420f9eb9bbf8d8d704c08caa4d8d4f4d4fc180b109aac351 /exec failed with exit code 1
• Run Regression Tests / test-nightly (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch --index-url https://downloa... / linux-job (gh)
  RuntimeError: Command docker exec -t b43ca455bd8e998b8e48eb5b971f7175ff008ae5d517ec06f9789aae798253fb /exec failed with exit code 1

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