LR Finder: compute gradients w.r.t. log10(lr) in exponential mode (fix torch.gradient spacing)

src/lightning/pytorch/CHANGELOG.md

@@ -46,6 +46,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed missing reset when `ModelPruning` is applied with lottery ticket hypothesis ([#21191](https://github.com/Lightning-AI/pytorch-lightning/pull/21191))
 
 
+- Fixed LR Finder: compute gradients w.r.t. `log10(lr)` in exponential mode #([#21171](https://github.com/Lightning-AI/pytorch-lightning/pull/21171))
+
+
 - Fixed preventing recursive symlink creation iwhen `save_last='link'` and `save_top_k=-1` ([#21186](https://github.com/Lightning-AI/pytorch-lightning/pull/21186))
 
 

src/lightning/pytorch/tuner/lr_finder.py

@@ -187,7 +187,8 @@ def suggestion(self, skip_begin: int = 10, skip_end: int = 1) -> Optional[float]
             self._optimal_idx = None
             return None
 
-        gradients = torch.gradient(losses, spacing=[lrs])[0]  # Compute the gradient of losses w.r.t. learning rates
+        spacing = [torch.log10(lrs)] if self.mode == "exponential" else [lrs]
+        gradients = torch.gradient(losses, spacing=spacing)[0]  # Compute the gradient of losses w.r.t. learning rates
         min_grad = torch.argmin(gradients).item()
         all_losses_idx = torch.arange(len(self.results["loss"]))
         idx_non_skipped = all_losses_idx[skip_begin:-skip_end]

tests/tests_pytorch/tuner/test_lr_finder.py

@@ -606,18 +606,21 @@ def configure_optimizers(self):
 
 
 def test_gradient_correctness():
-    """Test that torch.gradient uses correct spacing parameter."""
+    """Test that gradients are computed with log-spaced learning rates in exponential mode."""
     lr_finder = _LRFinder(mode="exponential", lr_min=1e-6, lr_max=1e-1, num_training=20)
 
-    # Synthetic example
-    lrs = torch.linspace(0, 2 * math.pi, steps=1000)
+    lrs = torch.logspace(-6, -1, steps=1000)
     losses = torch.sin(lrs)
     lr_finder.results = {"lr": lrs.tolist(), "loss": losses.tolist()}
 
-    # Test the suggestion method
     suggestion = lr_finder.suggestion(skip_begin=2, skip_end=2)
     assert suggestion is not None
-    assert abs(suggestion - math.pi) < 1e-2, "Suggestion should be close to pi for this synthetic example"
+
+    losses_t = torch.tensor(lr_finder.results["loss"][2:-2])
+    lrs_t = torch.tensor(lr_finder.results["lr"][2:-2])
+    gradients = torch.gradient(losses_t, spacing=[torch.log10(lrs_t)])[0]
+    expected_idx = torch.argmin(gradients).item() + 2
+    assert math.isclose(suggestion, lrs[expected_idx].item())
 
 
 def test_lr_finder_callback_applies_lr_after_restore(tmp_path):
@@ -738,15 +741,12 @@ def __init__(self):
         lrs_filtered = lrs[is_finite]
 
         if len(losses_filtered) >= 2:
-            # Test that gradient computation works and produces finite results
-            gradients = torch.gradient(losses_filtered, spacing=[lrs_filtered])[0]
+            spacing = [torch.log10(lrs_filtered)] if mode == "exponential" else [lrs_filtered]
+            gradients = torch.gradient(losses_filtered, spacing=spacing)[0]
             assert torch.isfinite(gradients).all(), f"Non-finite gradients in {mode} mode"
             assert len(gradients) == len(losses_filtered)
 
-            # Verify gradients with spacing differ from gradients without spacing
             gradients_no_spacing = torch.gradient(losses_filtered)[0]
-
-            # For exponential mode, these should definitely be different, for linear mode, they might be similar
             if mode == "exponential":
                 assert not torch.allclose(gradients, gradients_no_spacing, rtol=0.1), (
                     "Gradients should differ significantly in exponential mode when using proper spacing"