Logging how vllm importance ratios are truncated/masked in GRPOTrainer

### Feature request

I think logging more values related to vllm importance ratios will be helpful, e.g. logging max/min/mean of raw importance ratios (before truncation/mask) and the fraction of importance samples being truncated/masked.

### Motivation

As of now, all the logged values related to vllm importance sampling correction are based on already truncated/masked values, so it is hard to see what the raw values look like and how many importance ratios are truncated/masked.

Where the importance ratios are truncated/masked:
https://github.com/huggingface/trl/blob/1850da5a3cbaeb043b483ae78986c43246501d40/trl/trainer/grpo_trainer.py#L1807-L1814

Where the values are logged:
https://github.com/huggingface/trl/blob/1850da5a3cbaeb043b483ae78986c43246501d40/trl/trainer/grpo_trainer.py#L1955-L1977

### Your contribution

~I can send a PR to add metrics using importance ratios before truncation/masking.~
I made a PR: #5243