⚠️ Raise `Exception` when trying to generate 0 tokens ⚠️
Author: danielkorat
Pull request: yes

# What does this PR do?

Currently, setting `max_new_tokens=0` produces 1 token instead of 0, and only a warning is produced.
To prevent unexpected patterns of generation, this warning should be changed to an `Exception`.

### Example:

```python
from transformers import AutoModelForCausalLM, AutoTokenizer
checkpoint = "bigcode/tiny_starcoder_py"
tokenizer = AutoTokenizer.from_pretrained("bigcode/tiny_starcoder_py")
model = AutoModelForCausalLM.from_pretrained("bigcode/tiny_starcoder_py")
inputs = tokenizer("def print_hello_world():", return_tensors="pt")
outputs = model.generate(**inputs,
                        pad_token_id=tokenizer.eos_token_id,
                        max_new_tokens=0)
print(f"Input length: {len(inputs['input_ids'][0])}")
print(f"Output length: {len(outputs[0])}")
```

### Output before fix:
```bash
/home/sdp/fix-zero-max-new-tokens/transformers/src/transformers/generation/utils.py:1136: 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(
Input length: 7
Output length: 8
```

### Output after fix:
```bash
Traceback (most recent call last):
  File "/home/sdp/fix-zero-max-new-tokens/test.py", line 8, in <module>
    outputs = model.generate(**inputs,
  File "/storage/sdp/anaconda3/envs/fix-zero-max-new-tokens/lib/python3.10/site-packages/torch/utils/_contextlib.py", line 115, in decorate_context
    return func(*args, **kwargs)
  File "/home/sdp/fix-zero-max-new-tokens/transformers/src/transformers/generation/utils.py", line 1396, in generate
    self._validate_generated_length(generation_config, input_ids_length, has_default_max_length)
  File "/home/sdp/fix-zero-max-new-tokens/transformers/src/transformers/generation/utils.py", line 1136, in _validate_generated_length
    raise ValueError(
ValueError: 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`.
```

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Comments:
Hi @gante 
Can you please have a look?
Not sure why the tests are failing.

@danielkorat the test is failing because it is poorly parameterized -- it raises the newly added exception. I'd like to ask you to try to fix the test parameterization (ping me if you get stuck :) )

> @danielkorat the test is failing because it is poorly parameterized -- it raises the newly added exception. I'd like to ask you to try to fix the test parameterization (ping me if you get stuck :) )

Hi @gante @amyeroberts, 
I made the requested changes and fixed the test above as well. 
This test is now failing and it seems to be unrelated to my changes:
```bash
_ FlaxWav2Vec2ModelTest.test_equivalence_pt_to_flax [FlaxWav2Vec2ForPreTraining] _
....
E   AssertionError: 1.1444092e-05 not less than or equal to 1e-05 : outputs.codevector_perplexity: Difference between PyTorch and Flax is 1.1444091796875e-05 (>= 1e-05).
```

It was probably a flaky test 😉 

Thanks @ArthurZucker, I changed the title

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_28621). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@ArthurZucker can you merge please?

Thanks for the PR 😉 

@danielkorat thank you for the fix 💛