Fix exo bench for transformers 5.x (#1168)

## Motivation
Prompt Sizer was broken as transformers 5.x tokenizers create
BatchEncodings which are essentially a dictionary of {input_ids: []}
instead of the list of input ids.

## Test Plan

### Manual Testing
Tested that exo bench runs as expected.

### Automated Testing
<!-- Describe changes to automated tests, or how existing tests cover
this change -->
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bench/exo_bench.py

@@ -241,6 +241,9 @@ class PromptSizer:
             ids = tokenizer.apply_chat_template(
                 messages, tokenize=True, add_generation_prompt=True
             )
+            # Fix for transformers 5.x
+            if hasattr(ids, "input_ids"):
+                ids = ids.input_ids
             return int(len(ids))
 
         return count_fn