fix layernorm grad sbp

[ShawnXuan]

Fix SBP settings for LayerNormGradOp to ensure correct gradient aggregation for gamma_diff and beta_diff

Changes

• Updated SBP strategy in LayerNormGradOp: Set gamma_diff and beta_diff to use PartialSum instead of Split to avoid dimension mismatches during distributed training.
• Added consistency check for begin_norm_axis and begin_params_axis: Enforce equality to ensure proper alignment of normalization and parameter dimensions.