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):

• 7/x mx cleanup: standardize nvfp4 config names #3398
• 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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