Do not reinstall llm-compressor when it is already installed

The FP8/FP4 compressed export calls install_llm_compressor() in the export
worker process to ensure the dependency is present. That in-process import can
fail even when llm-compressor is installed, because Unsloth's transformers
patches interfere in this process (the actual quantization runs in a separate
clean subprocess that re-imports it fine). On that spurious failure the code
fell through to a pip install of the version-capped, torch/transformers-pinned
spec, which made pip re-resolve the whole graph and backtrack over every
llm-compressor release, eventually trying to build numpy<2 from source and
failing the export with a confusing pip error.

Guard the install on importlib.metadata: when a llm-compressor distribution is
already present, return instead of reinstalling. Reinstalling cannot fix an
in-process import error, and the isolated quantization subprocess imports it
cleanly. A genuinely absent install still triggers the auto-install as before.

unsloth/save.py

@@ -1383,6 +1383,23 @@ def install_llm_compressor():
     except Exception:
         pass
 
+    # Already installed but not importable in THIS process? Do not reinstall. The in-process import
+    # can fail under Unsloth's transformers patches (the compressed export quantizes in an isolated
+    # subprocess that re-imports cleanly), and reinstalling the version-capped, torch/transformers-
+    # pinned spec makes pip re-resolve and backtrack destructively (it drags in numpy<2 built from
+    # source and the export fails). Only fall through to a pip install when it is genuinely absent.
+    try:
+        from importlib.metadata import version as _iv, PackageNotFoundError as _PNF
+        try:
+            _iv("llmcompressor")
+            # Present; the compressed-export subprocess performs the real import. The caller only
+            # uses this to trigger the install and fail fast, so returning None here is safe.
+            return None, None
+        except _PNF:
+            pass
+    except Exception:
+        pass
+
     # Opt-out for locked-down / air-gapped setups: forbid the auto-install, require a manual one.
     if os.environ.get("UNSLOTH_DISABLE_LLM_COMPRESSOR_AUTOINSTALL", "0").lower() not in (
         "0",