[ft_attention] Fix for seqlen=8136 (#488)

When seqlen=8136, `smem_sz = 48840`, and apparently starting the kernel returns an `invalid argument` CUDA error.

`48840 < 48 * 1024` but apparently it's still above the limit somehow..?
Tested on A100

csrc/ft_attention/decoder_masked_multihead_attention.cu

@@ -31,9 +31,7 @@
     size_t smem_sz = mmha::smem_size_in_bytes<T, DO_CROSS_ATTENTION>(params, THDS_PER_VALUE, THDS_PER_BLOCK);          \
     auto kernel = mmha::masked_multihead_attention_kernel<T, Dh, Dh_MAX, THDS_PER_KEY, THDS_PER_VALUE,                 \
                                                           THDS_PER_BLOCK, DO_CROSS_ATTENTION>;                         \
-    if (smem_sz >= 48 * 1024) {                                                                                        \
     cudaFuncSetAttribute(kernel, cudaFuncAttributeMaxDynamicSharedMemorySize, smem_sz);                                \
-    }                                                                                                                  \
     dim3 grid(params.nnz_head_idx == nullptr ? params.num_heads : params.nnz_heads, params.batch_size);                \
     kernel<<<grid, THDS_PER_BLOCK, smem_sz, stream>>>(params)