skip fp8 CI on non-H100 GPUs (pytorch#465)

skip fp8 tests on non-H100 GPUs by checking
`torch.cuda.get_device_capability() >= (9, 0)`

this makes 4 GPU CI healthy again

torchtitan/float8_linear.py

@@ -13,10 +13,12 @@
 # Note: Performance
 # Float8 experimental is intended to be ran under `torch.compile`` for competitive performance
 import contextlib
+import functools
 from typing import Optional
 
 import torch
 import torch.nn as nn
+from torch._logging import warning_once
 
 from torchtitan.config_manager import JobConfig
 from torchtitan.logging_utils import logger
@@ -36,7 +38,13 @@ def set_enable_fsdp_fp8_all_gather(enable_fsdp_fp8_all_gather: bool):
         config.enable_fsdp_fp8_all_gather = prev
 
 
-def build_fp8_linear(
+@functools.lru_cache(None)
+def is_sm90_or_later():
+    # Float8 is only supported on H100+ GPUs
+    return torch.cuda.is_available() and torch.cuda.get_device_capability() >= (9, 0)
+
+
+def maybe_build_fp8_linear(
     model: nn.Module, job_config: JobConfig, dp_enabled: Optional[bool] = False
 ):
     """
@@ -46,16 +54,24 @@ def build_fp8_linear(
     This will mutate the model inplace.
     """
     enable_fp8_linear = job_config.training.enable_fp8_linear
-    enable_fsdp_fp8_all_gather = (
-        job_config.training.enable_fsdp_fp8_all_gather and dp_enabled
-    )
+    if not enable_fp8_linear:
+        return
+    if not is_sm90_or_later():
+        warning_once(
+            logger,
+            "Failed to swap to Float8Linear because SM90 or later is not available",
+        )
+        return
     try:
         from float8_experimental.float8_linear import TensorScalingType
         from float8_experimental.float8_linear_utils import (
             swap_linear_with_float8_linear,
         )
 
         # Mutates the model inplace replacing instances of torch.nn.Linear with Float8Linear
+        enable_fsdp_fp8_all_gather = (
+            job_config.training.enable_fsdp_fp8_all_gather and dp_enabled
+        )
         with set_enable_fsdp_fp8_all_gather(enable_fsdp_fp8_all_gather):
             swap_linear_with_float8_linear(
                 model, scaling_type_w=TensorScalingType.DYNAMIC
@@ -67,3 +83,23 @@ def build_fp8_linear(
         raise ImportError(
             "float8_experimental is not installed. Please install it to use fp8 linear layers."
         ) from exc
+
+
+def maybe_precompute_fp8_dynamic_scale_for_fsdp(
+    model: nn.Module, job_config: JobConfig
+):
+    if not (
+        job_config.training.enable_fp8_linear
+        and job_config.training.enable_fsdp_fp8_all_gather
+        and job_config.training.precompute_float8_dynamic_scale_for_fsdp
+    ):
+        return
+    if not is_sm90_or_later():
+        warning_once(
+            logger,
+            "Skipped precomputing fp8 scales because SM90 or later is not available",
+        )
+        return
+    from float8_experimental.fsdp_utils import precompute_float8_dynamic_scale_for_fsdp
+
+    precompute_float8_dynamic_scale_for_fsdp(model)

train.py

@@ -27,7 +27,10 @@
 from torchtitan.checkpoint import CheckpointManager
 from torchtitan.config_manager import JobConfig
 from torchtitan.datasets import build_hf_data_loader, create_tokenizer
-from torchtitan.float8_linear import build_fp8_linear
+from torchtitan.float8_linear import (
+    maybe_build_fp8_linear,
+    maybe_precompute_fp8_dynamic_scale_for_fsdp,
+)
 from torchtitan.logging_utils import init_logger, logger
 from torchtitan.lr_scheduling import get_lr_schedulers
 from torchtitan.metrics import build_gpu_memory_monitor, build_metric_logger
@@ -215,9 +218,8 @@ def loss_fn(pred, labels):
     with torch.device("meta"):
         whole_model = model_cls.from_model_args(model_config)
 
-    # apply fp8 linear module swap
-    if job_config.training.enable_fp8_linear:
-        build_fp8_linear(whole_model, job_config, parallel_dims.dp_enabled)
+    # swap to Float8Linear base on fp8 config
+    maybe_build_fp8_linear(whole_model, job_config, parallel_dims.dp_enabled)
 
     # log model size
     model_param_count = get_num_params(whole_model)
@@ -398,18 +400,10 @@ def loss_fn(pred, labels):
             optimizers.step()
             lr_schedulers.step()
 
-            if (
-                job_config.training.enable_fp8_linear
-                and job_config.training.enable_fsdp_fp8_all_gather
-                and job_config.training.precompute_float8_dynamic_scale_for_fsdp
-            ):
-                from float8_experimental.fsdp_utils import (
-                    precompute_float8_dynamic_scale_for_fsdp,
-                )
-
-                # calculate float8 dynamic amax/scale for all-parameter for FSDP2
-                # it issues a single all-reduce for all parameters at once for better performance
-                precompute_float8_dynamic_scale_for_fsdp(model)
+            # when fp8 config is on,
+            # calculate float8 dynamic amax/scale for all-parameter for FSDP2
+            # it issues a single all-reduce for all parameters at once for better performance
+            maybe_precompute_fp8_dynamic_scale_for_fsdp(model, job_config)
 
             losses_since_last_log.append(loss)