Fix dwq: check for actual safetensors in target_dir

[micuentadecasa]

Bug Description

mlx_lm.dwq silently skips target computation when --target-dir is empty but exists.
When the directory exists with no .safetensors files, the model is not loaded (model = None),
target computation is skipped, and then mx.load() crashes at runtime with:

RuntimeError: [load_safetensors] Failed to open file .../train/0000000000.safetensors                                     

This happens when a user creates the target directory ahead of time, or when a previous run
was interrupted before writing targets.

Closes #1159

Root Cause

In mlx_lm/quant/dwq.py line 317, the has_targets check uses target_dir.exists():

has_targets = target_dir.exists()                                                                                         

This returns True for any existing directory, regardless of whether it contains target files.
The subsequent code paths skip both model loading (line 334-335: model = None) and target
computation (line 338), then try to load nonexistent files at line 356.

Fix

• Replace target_dir.exists() with a check that verifies actual .safetensors files exist
  in both train/ and valid/ subdirectories
• Before: has_targets = target_dir.exists()
• After:

  has_targets = (                                                                                                         
      target_dir.is_dir()                                                                                                 
      and any((target_dir / "train").glob("*.safetensors"))                                                               
      and any((target_dir / "valid").glob("*.safetensors"))                                                               
  )                                                                                                                       

Validation

6 new unit tests in tests/test_dwq.py:

• Empty existing dir → has_targets = False (was True)
• train/ only → has_targets = False (was True)
• valid/ only → has_targets = False (was True)
• Both splits present → has_targets = True (unchanged)
• Nonexistent dir → has_targets = False (unchanged)
• Non-safetensors files → has_targets = False (was True)

$ python -m unittest tests.test_dwq -v                                                                                    
test_empty_dir_not_treated_as_has_targets ... ok                                                                          
test_dir_with_train_only_not_treated_as_has_targets ... ok                                                                
test_dir_with_valid_only_not_treated_as_has_targets ... ok                                                                
test_dir_with_both_splits_treated_as_has_targets ... ok                                                                   
test_nonexistent_dir_not_treated_as_has_targets ... ok                                                                    
test_dir_with_non_safetensors_files_not_treated_as_has_targets ... ok                                                     
                                                                                                                          
Ran 6 tests in 0.004s OK                                                                                                  

                                                                                                                          
---                                                                                                                       
                                                                                                                          
**Branch name:** `lc/fix-dwq-target-dir-empty-check`                                                                      
                                                                                                                          
**Files to include in the PR:**                                                                                           
- `mlx_lm/quant/dwq.py` (the fix)                                                                                         
- `tests/test_dwq.py` (new, 6 unit tests)                                                                                 
- `.gitignore` (add `test_temp/`)     

[angeloskath]

Looks good thanks!

I removed the test as it wasn't really testing the dwq code.