switch float8 logic from Float8DynamicLinear to Float8Linear (#436)

Summary:

After pytorch-labs/float8_experimental#300,
`Float8Linear` with default settings is equivalent to
`Float8DynamicLinear`. This PR changes `torchtitan` to use
`Float8Linear`.

To support the new UX of `float8_experimental` better, I also switched
the `fp8_linear` configuration to be a boolean on whether to swap the
linears or not. In the future we can add new options on how to configure
each linear (scaling type, scaling granularity, etc) - saving that for a
future PR.

Test Plan:

```
// run baseline (Float8DynamicLinear) for llama3_8b for 50 iterations on 4 GPUs,
// verify performance and loss values do not change meaningfully between
// baseline and this PR

// baseline (before this PR)
// 1. compile, bf16
// 2. compile, float8
// 3. compile, float8, fdsp_fp8_allgather=True
// 4. compile, float8, fdsp_fp8_allgather=True, tp=2
// logs: https://gist.github.com/vkuzo/e6d5f3b15349862bfad3706baad8c9ce

// experiment (this PR): repeat all of the above, but with Float8Linear
// logs: https://gist.github.com/vkuzo/a4d6754358facffa64df931654459631
```

Reviewers:

Subscribers:

Tasks:

Tags:

Author: vkuzo

torchtitan/config_manager.py

@@ -339,15 +339,11 @@ def __init__(self):
         )
         self.parser.add_argument(
             "--training.fp8_linear",
-            type=str,
-            default="",
-            choices=[
-                "dynamic",
-                "",
-            ],  # TODO: add "delayed" option back in when supported
+            action="store_true",
             help="""
-                Type of fp8 linear quantization to apply to the model ['', 'dynamic'].
-                This features requires you to install 'float8_experimental' which can be found
+                If true, swaps `torch.nn.Linear` with `Float8Linear` with
+                default settings (dynamic scaling).
+                This feature requires you to install 'float8_experimental' which can be found
                 here: https://github.com/pytorch-labs/float8_experimental
             """,
         )

torchtitan/float8_linear.py

@@ -21,34 +21,22 @@
 
 def build_fp8_linear(model: nn.Module, job_config: JobConfig):
     """
-    This function converts the linear layers to one of the fp8 types:
-    - Float8DynamicLinear: Dynamic quantization of the weights and the activations
-    - [Not Yet Supported] Float8Linear: Uses a history of amaxs to quantize the weights and activations
+    This function converts the linear layers to `Float8Linear`. Note that today,
+    only dynamic tensor scaling (the default) is supported.
 
     This will mutate the model inplace.
     """
-    linear_type = job_config.training.fp8_linear.lower()
+    use_fp8_linear = job_config.training.fp8_linear
     try:
-        from float8_experimental.float8_dynamic_linear import Float8DynamicLinear
-
-        # from float8_experimental.float8_linear import Float8Linear
+        from float8_experimental.float8_linear import Float8Linear
         from float8_experimental.float8_linear_utils import (
             swap_linear_with_float8_linear,
         )
     except ImportError as exc:
         raise ImportError(
             "float8_experimental is not installed. Please install it to use fp8 linear layers."
         ) from exc
-    if linear_type:
-        linear_type_map = {
-            # "delayed": Float8Linear, # TODO: add "delayed" option back in when supported
-            "dynamic": Float8DynamicLinear,
-        }
-        assert (
-            linear_type in linear_type_map
-        ), f"Invalid fp8 linear type: {linear_type}, supported types: {', '.join(linear_type_map.keys())}."
-        float8_linear_type = linear_type_map[linear_type.lower()]
-
-        # Mutates the model inplace replacing instances of torch.nn.Linear with float8_linear_type
-        swap_linear_with_float8_linear(model, float8_linear_type)
-        logger.info(f"Swapped to {linear_type} float8 linear layers")
+    if use_fp8_linear:
+        # Mutates the model inplace replacing instances of torch.nn.Linear with Float8Linear
+        swap_linear_with_float8_linear(model, Float8Linear)
+        logger.info("Swapped to Float8Linear layers")

train_configs/debug_model.toml

@@ -37,7 +37,7 @@ max_norm = 1.0  # grad norm clipping
 steps = 10
 data_parallel_degree = -1
 tensor_parallel_degree = 1
-fp8_linear = ""
+fp8_linear = false
 compile = false
 dataset = "c4_mini"  # supported datasets: c4_mini (45K), c4 (177M)
 

train_configs/llama2_13b.toml

@@ -33,7 +33,7 @@ max_norm = 1.0  # grad norm clipping
 steps = 1000
 data_parallel_degree = -1
 tensor_parallel_degree = 1
-fp8_linear = ""
+fp8_linear = false
 compile = false
 dataset = "c4"
 

train_configs/llama2_70b.toml

@@ -33,7 +33,7 @@ max_norm = 1.0  # grad norm clipping
 steps = 1000
 data_parallel_degree = -1
 tensor_parallel_degree = 8  # 8-way TP
-fp8_linear = ""
+fp8_linear = false
 compile = false
 dataset = "c4"
 

train_configs/llama2_7b.toml

@@ -32,7 +32,7 @@ max_norm = 1.0  # grad norm clipping
 steps = 1000
 data_parallel_degree = -1
 tensor_parallel_degree = 1  # dp-only would be sufficient for 7B
-fp8_linear = ""
+fp8_linear = false
 compile = false
 dataset = "c4"
 

train_configs/llama3_70b.toml

@@ -33,7 +33,7 @@ max_norm = 1.0  # grad norm clipping
 steps = 1000
 data_parallel_degree = -1
 tensor_parallel_degree = 8  # 8-way TP
-fp8_linear = ""
+fp8_linear = false
 compile = false
 dataset = "c4"
 

train_configs/llama3_8b.toml

@@ -33,7 +33,7 @@ max_norm = 1.0  # grad norm clipping
 steps = 1000
 data_parallel_degree = -1
 tensor_parallel_degree = 1
-fp8_linear = ""
+fp8_linear = false
 compile = false
 dataset = "c4"