only 1 thread writes to global mem in fprop

Signed-off-by: Kirthi Shankar Sivamani <ksivamani@nvidia.com>

csrc/flash_attn/src/fmha_fprop_kernel_1xN.h

@@ -667,6 +667,8 @@ inline __device__ void device_1xN_loop(const Params &params) {
     const int bidb = blockIdx.x;
     // The block index for the head.
     const int bidh = blockIdx.y;
+    // The block index.
+    const int bidx = gridDim.x * bidh + bidb;
     // The thread index.
     const int tidx = threadIdx.x;
 
@@ -678,8 +680,10 @@ inline __device__ void device_1xN_loop(const Params &params) {
     // the attention matrix. This way, as long as we have the batch, head, and the location of
     // the 16 x 16 block within the attention matrix, we can generate the exact same dropout pattern.
     auto seeds = at::cuda::philox::unpack(params.philox_args);
+    if (bidx == 0 && tidx == 0) {
         params.rng_state[0] = std::get<0>(seeds);
         params.rng_state[1] = std::get<1>(seeds);
+    }
     Philox ph(std::get<0>(seeds), 0, std::get<1>(seeds) + (bidb * params.h + bidh) * 32 + tidx % 32);
     constexpr int M = Kernel_traits::Cta_tile_p::M;
     const int STEPS = (params.seqlen_q + M - 1) / M;