[Bugfix] Fix bias for 0-dim tensors in gemm (#1246)

* fix bias for 0-dim tensor
Signed-off-by: Xin Yao <xiny@nvidia.com>

* add check
Signed-off-by: Xin Yao <xiny@nvidia.com>

* use numel() instead of nullptr
Signed-off-by: Xin Yao <xiny@nvidia.com>

---------
Signed-off-by: Xin Yao <xiny@nvidia.com>

transformer_engine/pytorch/csrc/extensions/gemm.cu

@@ -15,10 +15,16 @@ void te_gemm(at::Tensor A, at::Tensor A_scale_inverse, transformer_engine::DType
              at::Tensor workspace, size_t workspaceSize, bool accumulate,
              bool use_split_accumulator, int math_sm_count) {
   using namespace transformer_engine;
-  if (A.data_ptr() == nullptr || B.data_ptr() == nullptr) {
-    if (D.data_ptr() != nullptr && !accumulate) D.zero_();
-    if (bias.data_ptr() != nullptr) bias.zero_();
-    if (pre_gelu_out.data_ptr() != nullptr) pre_gelu_out.zero_();
+  if (A.numel() == 0 || B.numel() == 0) {
+    if (D.numel() != 0 && !accumulate) D.zero_();
+    if (bias.numel() != 0 && grad) {
+      if (B.numel() == 0) {
+        bias.zero_();
+      } else {
+        bias.copy_(B.sum(0));
+      }
+    }
+    if (pre_gelu_out.numel() != 0) pre_gelu_out.zero_();
     return;
   }
 
@@ -109,10 +115,16 @@ void te_grouped_gemm(std::vector<at::Tensor> A, at::Tensor A_scale_inverse, int
     return tensor_wrappers.back().data();
   };
   for (size_t i = 0; i < A.size(); i++) {
-    if (A[i].data_ptr() == nullptr || B[i].data_ptr() == nullptr) {
-      if (D[i].data_ptr() != nullptr && !accumulate) D[i].zero_();
-      if (bias[i].data_ptr() != nullptr) bias[i].zero_();
-      if (pre_gelu_out[i].data_ptr() != nullptr) pre_gelu_out[i].zero_();
+    if (A[i].numel() == 0 || B[i].numel() == 0) {
+      if (D[i].numel() != 0 && !accumulate) D[i].zero_();
+      if (bias[i].numel() != 0 && grad) {
+        if (B[i].numel() == 0) {
+          bias[i].zero_();
+        } else {
+          bias[i].copy_(B[i].sum(0));
+        }
+      }
+      if (pre_gelu_out[i].numel() != 0) pre_gelu_out[i].zero_();
       continue;
     }
 
@@ -175,6 +187,8 @@ void te_grouped_gemm_single_output(
   void* d_i_ptr = reinterpret_cast<void*>(D.data_ptr());
   for (size_t i = 0; i < A.size(); i++) {
     if (m_splits[i] == 0) continue;
+    NVTE_CHECK(A[i].data_ptr() != nullptr, "A[", i, "] must not be nullptr.");
+    NVTE_CHECK(B[i].data_ptr() != nullptr, "B[", i, "] must not be nullptr.");
     NVTE_CHECK(A[i].is_contiguous(), "A[", i, "] must be contiguous.");
     NVTE_CHECK(B[i].is_contiguous(), "B[", i, "] must be contiguous.");
     te_A.emplace_back(make_tensor(