Tokenizer updates + tests (#1024)

* tokenizer updates + tests

* nit

* add can_trim_prompt_cache

* nits

llms/mlx_lm/models/cache.py

@@ -77,6 +77,13 @@ def load_prompt_cache(file_name, return_metadata=False):
     return cache
 
 
+def can_trim_prompt_cache(cache: List[Any]) -> bool:
+    """
+    Check if model's cache can be trimmed.
+    """
+    return all(c.is_trimmable() for c in cache)
+
+
 def trim_prompt_cache(cache: List[Any], num_tokens: int) -> List[Any]:
     """
     Trim the model's cache by the given number of tokens.
@@ -91,7 +98,7 @@ def trim_prompt_cache(cache: List[Any], num_tokens: int) -> List[Any]:
     Returns:
         (int): The number of tokens that were trimmed.
     """
-    if not all(c.is_trimmable() for c in cache) or len(cache) == 0:
+    if not can_trim_prompt_cache(cache) or len(cache) == 0:
         return 0
     return [c.trim(num_tokens) for c in cache][0]