Fix some nits for layout

[LoserCheems]

This pull request simplifies the implementation of the compute_attn_1rowblock function in the flash_attention_fwd_kernel.h file by removing support for the causal mask feature. The changes streamline the code and improve maintainability by eliminating unused variables, memory allocations, and operations related to the causal mask.

Removal of Causal Mask Support:

• Removed all references to the causal mask, including memory allocation (sCausalMask), global memory tile (gCausalMask), and thread slice partitioning (gmem_thr_copy_CausalMask). [1] [2] [3] [4] [5]

• Deleted logic for handling the causal mask in shared memory, including initialization, copying, and synchronization steps.

Code Simplification:

• Adjusted shared memory layout and memory copy operations to focus solely on the remaining features, such as ZeroHold. [1] [2]

• Updated predicates and identity tensors to remove any references to the causal mask, ensuring the code reflects the simplified functionality. [1] [2]