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_paged_prefill.cu

@@ -24,7 +24,7 @@ torch::Tensor attn_paged_prefill(
                                     max_q_len, causal_offset, scale, p);
 
     auto O = torch::empty({q.size(0), q.size(1), q.size(2)}, q.options());
-    p.o = (bf16*)O.data_ptr();
+    p.o_ptr = (bf16*)O.data_ptr();
 
     DISPATCH_HEAD_DIM(p.head_dim, dispatch_paged_prefill, p, stream);
     C10_CUDA_CHECK(cudaGetLastError());