fix: dataset path should be absolute
cache_file_name depends on dataset's path.

A simple way where this could cause a problem:
```
import os
import datasets


def add_prefix(example):
    example["text"] = "Review: " + example["text"]
    return example


ds = datasets.load_from_disk("a/relative/path")
os.chdir("/tmp")

ds_1 = ds.map(add_prefix)
```

while it may feel that the `chdir` is quite constructed, there are many scenarios when the current working dir can/will change...