fix(server): avoid errors for very small top_p values (#544)

See https://github.com/huggingface/transformers/pull/24111

I didn't add validation to the `__init__` method since it's not done for
other values/warpers.

server/text_generation_server/utils/logits_process.py

@@ -189,7 +189,6 @@ class HeterogeneousTopPLogitsWarper(LogitsWarper):
 
         # Remove tokens with cumulative top_p above the threshold (token with 0 are kept)
         sorted_indices_to_remove = probs <= self.top_p_opposite
-        if self.min_tokens_to_keep > 1:
         # Keep at least min_tokens_to_keep
         sorted_indices_to_remove[..., -self.min_tokens_to_keep :] = 0