Fix ART Megatron installer uninstall step

Author: Kovbo

src/art_megatron_install.py

@@ -137,6 +137,14 @@ def _specifier_matches(requirement: Requirement) -> bool:
     return requirement.specifier.contains(Version(installed_version), prereleases=True)
 
 
+def _package_installed(name: str) -> bool:
+    try:
+        metadata.version(name)
+    except metadata.PackageNotFoundError:
+        return False
+    return True
+
+
 def dependencies_in_sync() -> bool:
     for requirement in _megatron_requirements().values():
         if requirement.url is not None:
@@ -204,7 +212,8 @@ def install_megatron_dependencies(*, dry_run: bool = False) -> None:
             dry_run=dry_run,
         )
 
-    _run([_uv_executable(), "pip", "uninstall", "-y", "pynvml"], dry_run=dry_run)
+    if dry_run or _package_installed("pynvml"):
+        _run([_uv_executable(), "pip", "uninstall", "pynvml"], dry_run=dry_run)
 
     remaining_requirements = [
         _format_requirement(requirement) for requirement in requirements.values()