Fix mask in _chunk_scan_chunk_state_bwd_dx that could cause NaN

Only affect cases where sequence length is not a multiple of 256

mamba_ssm/ops/triton/ssd_combined.py

@@ -170,7 +170,11 @@ def _chunk_scan_chunk_state_bwd_dx_kernel(
         dout = tl.load(dout_ptrs, mask=(offs_k[:, None] < K_MAX - k) & (offs_n[None, :] < hdim), other=0.0)
         dA_cs_k = tl.load(dA_cumsum_ptrs, mask=offs_k < K_MAX - k, other=0.0).to(tl.float32)
         cb *= tl.exp(dA_cs_k[None, :] - dA_cs_m[:, None])
-        mask = k + offs_k[None, :] >= offs_m[:, None]
+        # If we don't have the (k + offs_k[None, :] < K_MAX) mask, for indices outside this range,
+        # we might have dA_cs_m = 0.0 and dA_cs_k very negative, and tl.exp will return inf.
+        # Multiplying with cb, which is 0.0 outside the range, will make the result NaN.
+        # This will cause NaN in acc, and hence NaN in dx and ddt.
+        mask = (k + offs_k[None, :] >= offs_m[:, None]) & (k + offs_k[None, :] < K_MAX)
         cb = tl.where(mask, cb, 0.0)
         cb = cb.to(dout_ptr.dtype.element_ty)
         acc += tl.dot(cb, dout)