sanity check ranges in freeze.py (#1686)

* sanity check ranges in freeze.py

this will catch problems earlier and more clearly.

in my case, it appears that deepspeed zero3 sets layer tensor shapes
to [0], which doesn't play well with automatically inferred ranges.
through a bit of luck, inverting ranges still appears to work correctly.

* simplify chained comparison

src/axolotl/utils/freeze.py

@@ -120,6 +120,9 @@ def _merge_ranges(
     processed_ranges = [
         (start, end if end is not None else layer_size) for start, end in given_ranges
     ]
+    for start, end in processed_ranges:
+        if start < 0 or end > layer_size > 0 or start >= end:
+            raise ValueError(f"invalid unfreeze range: start={start}, end={end}")
 
     # No need to merge if there's only one or no ranges
     if len(processed_ranges) <= 1: