convert: fix converter crashes on Windows (fcntl import, null config) (#185)

Three crash bugs in convert_fp8_to_int4.py, all on the download path:

P3: 'import fcntl' at line 211 is Unix-only — ModuleNotFoundError on Windows.
    The --indir test path returns before reaching it so tests pass, but
    '--repo' on Windows hard-crashes. Guarded the import: try fcntl (Unix),
    fall back to msvcrt.locking (Windows), skip if neither available.

P4: repo_info retry loop had range(999) — up to ~16 hours of retries on a
    bad network, then fell through to line 395 where 'info' was unbound
    (NameError). Capped at 10 retries and added an explicit error + return
    when exhausted. Also added an early return if no safetensors shards are
    found in the repo.

P5: if the shards list was empty (wrong repo, all filtered out), the
    'for i, sh in enumerate(shards)' loop never executed and 'i' was unbound
    at line 460 (NameError). Now caught by the early return from P4.

Co-authored-by: woolcoxm <13604288+woolcoxm@users.noreply.github.com>

c/tools/convert_fp8_to_int4.py

@@ -208,11 +208,21 @@ def main():
 
     # lock anti-doppione: DUE convertitori sulla stessa outdir si corrompono a vicenda.
     # EN: anti-duplicate lock: TWO converters on the same outdir corrupt each other.
-    import fcntl
+    # fcntl is Unix-only; on Windows use msvcrt or skip locking.
     lock = open(os.path.join(a.outdir, ".convert.lock"), "w")
-    try: fcntl.flock(lock, fcntl.LOCK_EX | fcntl.LOCK_NB)
-    except OSError:
-        print("ERROR: another converter is already using this output directory. Exiting."); return
+    try:
+        import fcntl
+        try: fcntl.flock(lock, fcntl.LOCK_EX | fcntl.LOCK_NB)
+        except OSError:
+            print("ERROR: another converter is already using this output directory. Exiting."); return
+    except ImportError:
+        try:
+            import msvcrt
+            try: msvcrt.locking(lock.fileno(), msvcrt.LK_NBLCK, 1)
+            except OSError:
+                print("ERROR: another converter is already using this output directory. Exiting."); return
+        except ImportError:
+            pass  # no locking available — single-user converter, acceptable
 
     # dimensioni note dei file, riempite dopo repo_info: il downloader multi-stream le usa
     # per calcolare i confini dei segmenti e per sapere quando un file e' completo.
@@ -372,7 +382,8 @@ def main():
 
     from safetensors.numpy import save_file
     import time as _t
-    for att in range(999):
+    info = None
+    for att in range(10):
         try:
             info = HfApi().repo_info(a.repo, files_metadata=True)
             # dimensioni note dallo store: abilitano il download multi-stream a segmenti.
@@ -382,7 +393,13 @@ def main():
         except KeyboardInterrupt: raise
         except Exception as ex:
             w = min(60, 5*(att+1)); print(f"repo_info failed ({type(ex).__name__}); retrying in {w}s", flush=True); _t.sleep(w)
+    if info is None:
+        print("ERROR: could not reach the repository after 10 retries. Check your network and repo name.", flush=True)
+        return
     shards = sorted(s.rfilename for s in info.siblings if s.rfilename.endswith(".safetensors"))
+    if not shards:
+        print("ERROR: no .safetensors shards found in this repository.", flush=True)
+        return
     for fn in ["config.json", "tokenizer.json", "tokenizer_config.json", "generation_config.json"]:
         try: shutil.copy(hf_hub_download(a.repo, fn, local_dir=a.outdir+"/_meta"), a.outdir)
         except Exception: pass