[autodoc] Parse `PEP 257` style docstring

Copied from https://github.com/rr-/docstring_parser/issues/71#issue-1318744037

> The approved [PEP 257](https://peps.python.org/pep-0257/#what-is-a-docstring) mentions the so-called "attribute docstrings", which are string literals in the line after where an attribute is defined. These kind of docstrings are supported by a several packages, for example sphinx's [autodoc](https://www.sphinx-doc.org/en/master/usage/extensions/autodoc.html) extension.

An example looks like below:

```python
from dataclasses import dataclass
from typing import Optional
from docstring_parser import parse_from_object

@dataclass
class RewardConfig:
"""
RewardConfig collects all training arguments related to the [`RewardTrainer`] class.
"""

max_length: Optional[int] = None
"""The maximum length of the sequences in the batch. This argument is required if you want to use the default data collator."""
gradient_checkpointing: Optional[bool] = True
"""If True, use gradient checkpointing to save memory at the expense of slower backward pass."""

doc = parse_from_object(RewardConfig)

print(doc.short_description)
print()
for i in doc.params:
print(f"{i.arg_name}, {i.type_name} (default - {i.default})\n {i.description}")
print()
```
```
RewardConfig collects all training arguments related to the [`RewardTrainer`] class.

max_length, Optional[int] (default - None)
The maximum length of the sequences in the batch. This argument is required if you want to use the default data collator.

gradient_checkpointing, Optional[bool] (default - True)
If True, use gradient checkpointing to save memory at the expense of slower backward pass.
```

This appears not supported in the `autodoc` feature of this repo. For example, in TRL our config is https://github.com/huggingface/trl/blob/d608fea0d107d4359f9c03a9d6dd434d292a9f50/trl/trainer/ppo_config.py, but its related docs does not show the documentation.