Fix top_n_tokens returning non-log probs for some models (#1023)

# What does this PR do?

I made an embarrassing mistake where I accidentally passed normal
softmax probabilities into `batch_top_tokens` for `CausalLM` and
`Seq2SeqLM`.

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@Narsil

server/text_generation_server/models/causal_lm.py

@@ -579,7 +579,7 @@ class CausalLM(Model):
         batch_top_token_ids, batch_top_token_logprobs = batch_top_tokens(
             batch.top_n_tokens,
             batch.top_n_tokens_tensor,
-            torch.softmax(logits[:, -1], -1),
+            torch.log_softmax(logits[:, -1], -1),
         )
 
         # Zipped iterator

server/text_generation_server/models/seq2seq_lm.py

@@ -642,7 +642,7 @@ class Seq2SeqLM(Model):
         batch_top_token_ids, batch_top_token_logprobs = batch_top_tokens(
             batch.top_n_tokens,
             batch.top_n_tokens_tensor,
-            torch.softmax(logits[:, -1], -1),
+            torch.log_softmax(logits[:, -1], -1),
         )
 
         # Finished requests