fix: set NUM_WORKERS=0 for non-fork multiprocessing start methods (#4784)

fix #4686 

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Bug Fixes**
- Improved compatibility by automatically adjusting worker settings when
the multiprocessing start method is not supported, preventing potential
issues with data loading.
- Added a warning message to inform users when worker settings are
changed due to multiprocessing constraints.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: caic99

deepmd/pt/utils/env.py

@@ -1,4 +1,6 @@
 # SPDX-License-Identifier: LGPL-3.0-or-later
+import logging
+import multiprocessing
 import os
 
 import numpy as np
@@ -22,6 +24,15 @@
 except AttributeError:
     ncpus = os.cpu_count()
 NUM_WORKERS = int(os.environ.get("NUM_WORKERS", min(4, ncpus)))
+if multiprocessing.get_start_method() != "fork":
+    # spawn or forkserver does not support NUM_WORKERS > 0 for DataLoader
+    log = logging.getLogger(__name__)
+    log.warning(
+        "NUM_WORKERS > 0 is not supported with spawn or forkserver start method. "
+        "Setting NUM_WORKERS to 0."
+    )
+    NUM_WORKERS = 0
+
 # Make sure DDP uses correct device if applicable
 LOCAL_RANK = os.environ.get("LOCAL_RANK")
 LOCAL_RANK = int(0 if LOCAL_RANK is None else LOCAL_RANK)