[docs][torch.compile] Add fusions.md — kernel/operator fusion reference page (#35538)

Signed-off-by: ProExpertProg <luka.govedic@gmail.com>
Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: ProExpertProg <11367180+ProExpertProg@users.noreply.github.com>
Co-authored-by: ProExpertProg <luka.govedic@gmail.com>
Co-authored-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Co-authored-by: Luka Govedič <ProExpertProg@users.noreply.github.com>

vllm/config/compilation.py

@@ -4,7 +4,7 @@
 import enum
 from collections import Counter
 from collections.abc import Callable
-from dataclasses import field
+from dataclasses import field, fields
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, ClassVar, Literal
 
@@ -269,6 +269,24 @@ class PassConfig:
             )
             self.fuse_rope_kvcache = False
 
+    def log_enabled_passes(self) -> None:
+        """
+        Log the enabled custom fusion passes.
+        This is called at the end of VLLMConfig post_init,
+        after all defaults are finalized.
+        TODO also log the compile ranges for which this is enabled.
+        """
+        enabled_fusions = [
+            f.name[len("fuse_") :]
+            for f in fields(self)
+            if getattr(self, f.name) and f.name.startswith("fuse_")
+        ]
+
+        if enabled_fusions:
+            logger.info_once(
+                "Enabled custom fusions: %s", ", ".join(enabled_fusions), scope="global"
+            )
+
 
 class DynamicShapesType(str, enum.Enum):
     """Types of dynamic shapes handling in torch.compile().
@@ -341,7 +359,8 @@ class CompilationConfig:
     VLLMConfig's post_init does further initialization. If used outside of the
     VLLMConfig, some fields will be left in an improper state.
 
-    It has three parts:
+    It contains PassConfig, which controls the custom fusion/transformation passes.
+    The rest has three parts:
 
     - Top-level Compilation control:
         - [`mode`][vllm.config.CompilationConfig.mode]

vllm/config/vllm.py

@@ -1272,6 +1272,9 @@ class VllmConfig:
         # Handle the KV connector configs
         self._post_init_kv_transfer_config()
 
+        # Log the custom passes that are enabled
+        self.compilation_config.pass_config.log_enabled_passes()
+
     def update_sizes_for_sequence_parallelism(self, possible_sizes: list) -> list:
         # remove the sizes that not multiple of tp_size when
         # enable sequence parallelism