[generation] Less verbose warnings by default
Author: gante
Pull request: yes

Carved out from #38129, with a few upgrades.

Reduces the verbosity of bad generation config attribute warnings by default, with an env variable to control verbosity. Although users might get unexpected results when these warnings exist, most of the time they are harmless, so let's be less verbose by default.

### Example

Consider the following script, which will throw a warning:
```py
from transformers import AutoModelForCausalLM, AutoTokenizer

model_name = "Qwen/Qwen3-0.6B"
model = AutoModelForCausalLM.from_pretrained(model_name, device_map="auto")
tokenizer = AutoTokenizer.from_pretrained(model_name)

prompt = "Tell me a story about a cat:"
inputs = tokenizer(prompt, return_tensors="pt").to(model.device)

# This will throw warnings: there are sample-related attributes set in qwen3's generation config
outputs = model.generate(**inputs, do_sample=False, max_new_tokens=1)
```

On main, we would get
```
/home/joao/transformers/src/transformers/generation/configuration_utils.py:636: UserWarning: `do_sample` is set to `False`. However, `temperature` is set to `0.6` -- this flag is only used in sample-based generation modes. You should set `do_sample=True` or unset `temperature`.
  warnings.warn(
/home/joao/transformers/src/transformers/generation/configuration_utils.py:641: UserWarning: `do_sample` is set to `False`. However, `top_p` is set to `0.95` -- this flag is only used in sample-based generation modes. You should set `do_sample=True` or unset `top_p`.
  warnings.warn(
/home/joao/transformers/src/transformers/generation/configuration_utils.py:658: UserWarning: `do_sample` is set to `False`. However, `top_k` is set to `20` -- this flag is only used in sample-based generation modes. You should set `do_sample=True` or unset `top_k`.
  warnings.warn(
```

With this PR, we get
```
The following generation flags are not valid and may be ignored: ['temperature', 'top_p', 'top_k']. Set `TRANSFORMERS_VERBOSITY=info` for more details.
```

If we run the script with `TRANSFORMERS_VERBOSITY=info`, we get
```
(... other non-generate info logs)
The following generation flags are not valid and may be ignored: ['temperature', 'top_p', 'top_k'].
- `temperature`: `do_sample` is set to `False`. However, `temperature` is set to `0.6` -- this flag is only used in sample-based generation modes. You should set `do_sample=True` or unset `temperature`.
- `top_p`: `do_sample` is set to `False`. However, `top_p` is set to `0.95` -- this flag is only used in sample-based generation modes. You should set `do_sample=True` or unset `top_p`.
- `top_k`: `do_sample` is set to `False`. However, `top_k` is set to `20` -- this flag is only used in sample-based generation modes. You should set `do_sample=True` or unset `top_k`.
If you're using a pretrained model, note that some of these attributes may be set through the model's `generation_config.json` file.
```

Comments:
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