add specilizations for VecMaskLoad and VecMaskCast

[CaoE]

Stack from ghstack (oldest at bottom):

• [inductor] Use full bit widths for bf16/fp16 vectorization #126502
• -> add specilizations for VecMaskLoad and VecMaskCast #126501
• add VectorizedN support for welford_vec_reduce_all and add float<->bf16/fp16 conversion specialization for VectorizedN #126500

cc @jgong5 @mingfeima @XiaobingSuper @sanchitintel @ashokei @jingxu10

[pytorch-bot]

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❌ 1 New Failure, 6 Unrelated Failures

As of commit 0f40c07 with merge base ab07867 ():

NEW FAILURE - The following job has failed:

• Check mergeability of ghstack PR / ghstack-mergeability-check (gh)
  RuntimeError: Command git -C /home/runner/work/pytorch/pytorch diff origin/gh/CaoE/34/orig^! returned non-zero exit code 128

BROKEN TRUNK - The following jobs failed but were present on the merge base:

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

• inductor-periodic / cuda12.1-py3.10-gcc9-sm86-periodic-dynamo-benchmarks / test (aot_eager_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu) (gh) (trunk failure)
  hf_BigBird
• inductor-periodic / cuda12.1-py3.10-gcc9-sm86-periodic-dynamo-benchmarks / test (dynamic_aot_eager_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu) (gh) (trunk failure)
  hf_BigBird
• inductor-periodic / cuda12.1-py3.10-gcc9-sm86-periodic-dynamo-benchmarks / test (dynamo_eager_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu) (gh) (trunk failure)
  hf_BigBird

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

• inductor / cuda12.1-py3.10-gcc9-sm86 / test (aot_inductor_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu, unstable) (gh) (#120841)
  hf_BigBird
• inductor / cuda12.1-py3.10-gcc9-sm86 / test (dynamic_inductor_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu, unstable) (gh) (#120841)
  hf_BigBird
• inductor / cuda12.1-py3.10-gcc9-sm86 / test (inductor_torchbench, 1, 2, linux.g5.4xlarge.nvidia.gpu, unstable) (gh) (#120841)
  hf_BigBird

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