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closes #65

Co-authored-by: Nick Hill <nickhill@us.ibm.com>

Reverts huggingface/text-generation-inference#36

Add token streaming using ServerSideEvents (SSE).

The signature of the SSE events is: 

```rust
struct Details {
    finish_reason: String,
    generated_tokens: u32,
    seed: Option<u64>,
}

struct StreamResponse {
    token: Token,
    generated_text: Option<String>,
    details: Option<Details>,
}

struct ErrorResponse {
    error: String,
}
```

I noticed some opportunity to collapse some of the logic, in case you
are interested.

There's currently a discrepancy in the tokenization between the router
and python server code. The latter includes special tokens but former
does not.

This results in a token count mismatch for seq2seq models such as mt0
where the tokenizer emits an EOS token at the end.

This in turn results in some unexpected/incorrect output, in particular
when batch concatenation is involved, because the python code uses the
input length passed from the router for each row.

As far as I can tell, it is better to include this token in the encoder
`input_ids`, so I guess it's best to just adjust on the router side.

- Avoid theoretical hang in batcher loop
- Avoid a couple of clones in the router generate method
- Keep attention mask tensors as integers
- Remove num_heads attribute
Co-authored-by: OlivierDehaene <Olivier.dehaene@gmail.com>

Added validation logic