Improve handling for FP8 grouped gemm without zero_start_index_M (#3615)

Summary:
Pull Request resolved: #3615

X-link: facebookresearch/FBGEMM#694

When zero_start_index_M isnt provided, inputs can have variable M values across groups. To support this, we need to return a tensor with shape [total_M, N] since it isnt possible to view the tensor as [G, M, N].

Reviewed By: jasonjk-park, bradleyhd

Differential Revision: D68686266

fbshipit-source-id: 4267de288d6f7f2d0dec82b881eba056c11ea737

fbgemm_gpu/experimental/gen_ai/src/quantize/ck_extensions/fp8_rowwise_grouped/fp8_rowwise_grouped_gemm.hip

@@ -481,15 +481,42 @@ at::Tensor f8f8bf16_rowwise_grouped_dynamic(
     TORCH_CHECK(ws.dtype() == at::kFloat, "Scales must be float32.");
   }
 
-  // Create a single chunk of tensor but view it as a list for compatibility.
+  at::Tensor Y_full;
+  std::vector<at::Tensor> Y;
   int M = XQ[0].size(0);
   int N = WQ[0].size(0);
-  // Allocate an empty output array. We will set its values to zero as part
-  // of kernel setup.
-  at::Tensor Y_full =
-      at::empty({group_count, M, N}, XQ[0].options().dtype(at::kBFloat16));
-  // Split the output into groups.
-  std::vector<at::Tensor> Y = at::unbind(Y_full, 0);
+  int K = XQ[0].size(1);
+  // When run with padding, we return an output with shape [G, M, N] since
+  // M and N are consistent across groups.
+  if (zero_start_index_M.has_value()) {
+    // Allocate an empty output array. We will set its values to zero as part
+    // of kernel setup.
+    Y_full =
+        at::empty({group_count, M, N}, XQ[0].options().dtype(at::kBFloat16));
+    // Split the output into groups.
+    Y = at::unbind(Y_full, 0);
+    // When padding isnt provided, we return a tensor with shape [Total_M, N]
+    // since viewing as groups isnt possible due to variable M.
+  } else {
+    int total_M = 0;
+    std::vector<int> group_sizes = {};
+    for (int i = 0; i < group_count; i++) {
+      TORCH_CHECK(
+          XQ[i].size(1) == K && WQ[i].size(0) == N && WQ[i].size(1) == K,
+          "Dynamic grouped gemm requires fixed N and K.");
+      int group_M = XQ[i].size(0);
+      total_M += group_M;
+      group_sizes.push_back(group_M);
+    }
+    // Allocate continuous array for all groups.
+    Y_full = at::empty({total_M, N}, XQ[0].options().dtype(at::kBFloat16));
+    // Split full array into groups for downstream handling.
+    int offset = 0;
+    for (int size : group_sizes) {
+      Y.push_back(Y_full.narrow(0, offset, size));
+      offset += size;
+    }
+  }
 
   // Prepare kernel arguments by copying them to the proper device location.
   at::Tensor kernel_args =