Update generate method - Fix floor_divide warning
# What does this PR do?

Starting with PyTorch 1.9, a HF translation model (or any generation model) gives the following warning message:
```
/home/reimers/miniconda3/envs/easynmt/lib/python3.8/site-packages/torch/_tensor.py:575: UserWarning: floor_divide is deprecated, and will be removed in a future version of pytorch. It currently rounds toward 0 (like the 'trunc' function NOT 'floor'). This results in incorrect rounding for negative values.
To keep the current behavior, use torch.div(a, b, rounding_mode='trunc'), or for actual floor division, use torch.div(a, b, rounding_mode='floor'). (Triggered internally at  /opt/conda/conda-bld/pytorch_1623448234945/work/aten/src/ATen/native/BinaryOps.cpp:467.)
  return torch.floor_divide(self, other)
```

Here the code to produce this warning:
```
import warnings
warnings.filterwarnings("error")   #Turn warning into an exception for traceback

from transformers import MarianTokenizer, MarianMTModel

model_name = 'Helsinki-NLP/opus-mt-de-en'
tokenizer = MarianTokenizer.from_pretrained(model_name)
model = MarianMTModel.from_pretrained(model_name)
model.eval()

inputs = tokenizer(["Hallo Welt"], return_tensors="pt")
translated = model.generate(**inputs, num_beams=3)
print(translated)
```

Here the responsible line:
https://github.com/huggingface/transformers/blob/a6d62aaba01ce4ff1b2ee8705bf113904672c345/src/transformers/generation_utils.py#L1838

The // operator is translated to floor_divide, which is deprecated starting PyTorch 1.9:
https://pytorch.org/docs/stable/generated/torch.floor_divide.html


We replace this line with:
```
next_indices = (next_tokens/vocab_size).long()  
```

which is is compatible with any PyTorch version and yields identical results to `next_tokens // vocab_size`.


Here a test to show this:
```
import random
import torch
for _ in range(100):
    a = torch.tensor([random.randint(1, 1000)])
    b = torch.tensor([random.randint(1, 100)])
    c = a // b
    d = (a/b).long()
    assert torch.equal(c,d)
```` 


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

@sgugger


According to the document of [`torch.div`,](https://pytorch.org/docs/stable/generated/torch.div.html#torch.div) it is more suitable to change `next_indices = next_tokens // vocab_size` to `next_indices = torch.div(next_tokens, vocab_size, rounding_mode='floor')`.

`"floor"` - rounds the results of the division down. Equivalent to floor division in Python (the // operator)<|||||>> According to the document of [`torch.div`,](https://pytorch.org/docs/stable/generated/torch.div.html#torch.div) it is more suitable to change `next_indices = next_tokens // vocab_size` to `next_indices = torch.div(next_tokens, vocab_size, rounding_mode='floor')`.
> 
> `"floor"` - rounds the results of the division down. Equivalent to floor division in Python (the // operator)


rounding_method was only introduced in pytorch 1.8. Using this method would break with any Pytorch version before 1.8.  `(next_tokens/vocab_size).long()` is compatible with any pytorch version.<|||||>This makes sense!