update float8 training readme to include time measurement (#2291)

Summary:

Update the float8 training example code snippet to include time
measurement that properly excludes torch.compile one-time warmup. Also,
use larger shapes to demonstrate speedup from float8.

Test Plan:

copy-paste the snippet and run it, it works. Commenting out float8 shows
a slowdown, as expected.

Reviewers:

Subscribers:

Tasks:

Tags:

Author: vkuzo

torchao/float8/README.md

@@ -17,6 +17,8 @@ and composable with key systems such as autograd, ```torch.compile``` and distri
 This is the default recipe, with a good balance of performance and accuracy.
 
 ```python
+import time
+
 import torch
 import torch.nn as nn
 from torchao.float8 import convert_to_float8_training
@@ -26,11 +28,12 @@ if not TORCH_VERSION_AT_LEAST_2_5:
     raise AssertionError("torchao.float8 requires PyTorch version 2.5 or greater")
 
 # create model and sample input
+M, K, N = 4096, 8192, 4096
 m = nn.Sequential(
-    nn.Linear(2048, 4096),
-    nn.Linear(4096, 128),
+    nn.Linear(K, N, bias=False),
+    nn.Linear(N, 128, bias=False),
 ).bfloat16().cuda()
-x = torch.randn(4096, 2048, device="cuda", dtype=torch.bfloat16)
+x = torch.randn(M, K, device="cuda", dtype=torch.bfloat16)
 optimizer = torch.optim.SGD(m.parameters(), lr=0.1)
 
 # optional: filter modules from being eligible for float8 conversion
@@ -50,12 +53,26 @@ convert_to_float8_training(m, module_filter_fn=module_filter_fn)
 # enable torch.compile for competitive performance
 m = torch.compile(m)
 
+# warm up torch.compile for a clean training time measurement
+for _ in range(1):
+    optimizer.zero_grad()
+    y = m(x)
+    y.sum().backward()
+    optimizer.step()
+
+torch.cuda.synchronize()
+start_time = time.time()
+
 # toy training loop
 for _ in range(10):
     optimizer.zero_grad()
     y = m(x)
     y.sum().backward()
     optimizer.step()
+
+torch.cuda.synchronize()
+end_time = time.time()
+print("Training time:", end_time - start_time)
 ```
 
 ## float8 linear with rowwise scaling