Fix: `generate()` with `max_new_tokens=0` produces a single token.
Author: danielkorat
Pull request: yes

# What does this PR do?

Currently, setting `max_new_tokens=0` produces 1 token instead of 0, and the warning is unclear.
For example, for the following code:

```python
checkpoint = "bigcode/tiny_starcoder_py"
tokenizer = AutoTokenizer.from_pretrained(checkpoint)
model = AutoModelForCausalLM.from_pretrained(checkpoint)
inputs = tokenizer("def print_hello_world():", return_tensors="pt")
max_new_tokens = 0
outputs = model.generate(**inputs,
                        pad_token_id=tokenizer.eos_token_id,
                        max_new_tokens=max_new_tokens)
input_length = len(inputs['input_ids'][0])
output_length = len(outputs[0])
print(f"\nTest:{input_length - output_length == max_new_tokens}")
```
The output is:
```bash
utils.py:1134: UserWarning: Input length of input_ids is 7, but `max_length` is set to 7. This can lead to unexpected behavior. You should consider increasing `max_new_tokens`.
  warnings.warn(

Test: False
```

After the fix, this is the output:
```bash
`max_new_tokens`=0, no tokens will be generated.
utils.py:1134: UserWarning: Input length of input_ids is 7, but `max_length` is set to 7. This can lead to unexpected behavior. You should consider increasing `max_new_tokens`.
  warnings.warn(

Test: True
```
(Note the new warning).

Currently fixed only for `greedy_search()`. Once this PR is reviewed, I'll add the fix to all other generation modes.
@gante @amyeroberts 

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Comments:
Hi @danielkorat 👋 

In lower-level APIs like `generate`, we should strive to be more strict if some parameterization is causing problems to avoid creating unexpected patterns. In this case, generating with 0 tokens should not be possible, as there is no generation involved. 

As such, I'd suggest increasing the severity of the [associated warning](https://github.com/huggingface/transformers/blob/b2748a6efd045dd771f8fd48e8b309cbc061c618/src/transformers/generation/utils.py#L1134) to an exception instead of the changes proposed here :)

> Hi @danielkorat 👋
> 
> In lower-level APIs like `generate`, we should strive to be more strict if some parameterization is causing problems to avoid creating unexpected patterns. In this case, generating with 0 tokens should not be possible, as there is no generation involved.
> 
> As such, I'd suggest increasing the severity of the [associated warning](https://github.com/huggingface/transformers/blob/b2748a6efd045dd771f8fd48e8b309cbc061c618/src/transformers/generation/utils.py#L1134) to an exception instead of the changes proposed here :)

Hi @gante, I made the requested change in a new PR: [28621](https://github.com/huggingface/transformers/pull/28621).