[float8] Re-enable slow-accum in the bwd of axis-wise scaling schemes

And circumvent the issue with the slow CUTLASS kernel by using the cuBLAS kernel + manual scaling.

ghstack-source-id: 2c15187facc188f316d233365098211abdf6a039
Pull Request resolved: #1325

torchao/float8/config.py

@@ -170,7 +170,6 @@ class Float8LinearConfig:
     #
     # Per-gemm configuration for gemms calculating `output`, `grad_input` and
     # `grad_weight`
-    # TODO(this PR): throw warning if fast_accum False is used with axiswise scaling
     #
     gemm_config_output: Float8GemmConfig = Float8GemmConfig(use_fast_accum=True)
     gemm_config_grad_input: Float8GemmConfig = Float8GemmConfig()
@@ -320,21 +319,10 @@ def recipe_name_to_linear_config(
         cc_w = CastConfig(scaling_granularity=ScalingGranularity.AXISWISE)
         cc_go = CastConfig(scaling_granularity=ScalingGranularity.AXISWISE)
 
-        # The current rowwise CUTLASS kernels in `torch._scaled_mm` are only
-        # fast with `use_fast_accum=True`. Note that rowwise scaling is more
-        # accurate than tensorwise scaling, so the overall impact on accuracy
-        # of tensorwise vs rowwise taking this flag into account will vary.
-        gc_o = Float8GemmConfig(use_fast_accum=True)
-        gc_gi = Float8GemmConfig(use_fast_accum=True)
-        gc_gw = Float8GemmConfig(use_fast_accum=True)
-
         return Float8LinearConfig(
             cast_config_input=cc_i,
             cast_config_weight=cc_w,
             cast_config_grad_output=cc_go,
-            gemm_config_output=gc_o,
-            gemm_config_grad_input=gc_gi,
-            gemm_config_grad_weight=gc_gw,
         )
 
     elif recipe_name is Float8LinearRecipeName.LW_AXISWISE_WITH_GW_HP:
@@ -362,24 +350,13 @@ def recipe_name_to_linear_config(
         cc_i_gw = CastConfig(scaling_type=ScalingType.DISABLED)
         cc_go_gw = CastConfig(scaling_type=ScalingType.DISABLED)
 
-        # The current rowwise CUTLASS kernels in `torch._scaled_mm` are only
-        # fast with `use_fast_accum=True`. Note that rowwise scaling is more
-        # accurate than tensorwise scaling, so the overall impact on accuracy
-        # of tensorwise vs rowwise taking this flag into account will vary.
-        gc_o = Float8GemmConfig(use_fast_accum=True)
-        gc_gi = Float8GemmConfig(use_fast_accum=True)
-        gc_gw = Float8GemmConfig(use_fast_accum=True)
-
         return Float8LinearConfig(
             cast_config_input=cc_i,
             cast_config_weight=cc_w,
             cast_config_grad_output=cc_go,
             cast_config_input_for_grad_weight=cc_i_gw,
             cast_config_weight_for_grad_input=cc_w_gi,
             cast_config_grad_output_for_grad_weight=cc_go_gw,
-            gemm_config_output=gc_o,
-            gemm_config_grad_input=gc_gi,
-            gemm_config_grad_weight=gc_gw,
         )
 
     else:

torchao/float8/float8_python_api.py

@@ -37,19 +37,21 @@ def addmm_float8_unwrapped(
     a_inverse_scale = a_scale.reciprocal()
     b_inverse_scale = b_scale.reciprocal()
 
-    if output_dtype == torch.float32 and bias is not None:
+    post_inverse_scale = None
+    if a_scale.shape == (a_data.shape[0], 1) and b_scale.shape == (1, b_data.shape[1]) and not use_fast_accum:
+        # The rowwise CUTLASS-based kernel is so slow without fast-accum that
+        # we'd rather use the tensorwise cuBLAS-based kernel and do the scaling
+        # manually afterwards (hoping Inductor will be able to fuse it).
+        post_inverse_scale = a_inverse_scale * b_inverse_scale
+        a_inverse_scale = a_inverse_scale.new_ones(())
+        b_inverse_scale = a_inverse_scale.new_ones(())
+
+    post_bias = None
+    if output_dtype == torch.float32:
         # Bias is not supported by _scaled_mm when output is fp32
-        output = torch._scaled_mm(
-            a_data,
-            b_data,
-            scale_a=a_inverse_scale,
-            scale_b=b_inverse_scale,
-            scale_result=output_scale,
-            out_dtype=output_dtype,
-            use_fast_accum=use_fast_accum,
-        )
-        output += bias
-        return output
+        post_bias = bias
+        bias = None
+
     output = torch._scaled_mm(
         a_data,
         b_data,
@@ -60,4 +62,10 @@ def addmm_float8_unwrapped(
         out_dtype=output_dtype,
         use_fast_accum=use_fast_accum,
     )
+
+    if post_inverse_scale is not None:
+        output *= post_inverse_scale
+    if post_bias is not None:
+        output += post_bias
+
     return output