fix: LlamaTokenizerFast to AutoTokenizer at flash_llama.py (#619)

# What does this PR do?

A few tokenizer_config in huggingface use LlamaTokenizer, so I think I
would have selected `LlamaTokenizer` before.

For a few cases where you're using a llama structure but not a llama
tokenizer, why not make it to call the AutoTokenizer in exception
handling.

In the case of `decapoda-research/llama-7b-hf`, LLamaTokenizer is still
being used in config.json, so it should be called through`
LlamaTokenizer`.
Also, if an exception is thrown by LlamaTokenizer, it will cause
`LlamaTokenzierFast` to be called from AutoTokenizer.


Fixes # 560


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@Narsil

server/text_generation_server/models/flash_llama.py

@@ -2,7 +2,8 @@ import torch
 import torch.distributed
 
 from opentelemetry import trace
-from transformers.models.llama import LlamaTokenizer, LlamaTokenizerFast
+from transformers import AutoConfig, AutoTokenizer
+from transformers.models.llama import LlamaTokenizer
 from typing import Optional
 
 from text_generation_server.models import FlashCausalLM
@@ -44,7 +45,7 @@ class FlashLlama(FlashCausalLM):
                 trust_remote_code=trust_remote_code,
             )
         except Exception:
-            tokenizer = LlamaTokenizerFast.from_pretrained(
+            tokenizer = AutoTokenizer.from_pretrained(
                 model_id,
                 revision=revision,
                 padding_side="left",