refactor: tidy attention params and launcher interfaces

- rename output pointer field o to o_ptr for consistency with q_ptr/k_ptr/v_ptr
- regroup AttentionParams fields by responsibility and fix misleading comments
- drop unused max_seq_len/total_q fields and paged decode max_seq_len arg
- drop redundant group_size param from decode launchers (computed from p)

csrc/kernels/attn_prefill.cu

@@ -19,7 +19,7 @@ torch::Tensor attn_prefill(
 
     auto O = torch::empty_strided(q.sizes(), q.strides(), q.options());
     auto O_view = (layout == BLHD) ? O.transpose(1, 2) : O;
-    p.o = (bf16*)O_view.data_ptr();
+    p.o_ptr = (bf16*)O_view.data_ptr();
 
     DISPATCH_HEAD_DIM(p.head_dim, dispatch_prefill, p, stream);
     C10_CUDA_CHECK(cudaGetLastError());