TqdmCallback: add support for custom tqdm subclasses

[skshetry]

This will allow customizing the tqdm progressbar used for the callback, and extending (since all tqdm_kwargs is passed to the underlying implementation.)

E.g., in DVC, we have custom logic to align all the progress bars of tqdm, even if they have different description lengths.

This PR will enable us to use TqdmCallback directly in our code, without writing our own implementation.

---

from tqdm import tqdm

class TqdmExtraFormat(tqdm):
    """Provides a `total_time` format parameter"""
    @property
    def format_dict(self):
        d = super(TqdmExtraFormat, self).format_dict
        total_time = d["elapsed"] * (d["total"] or 0) / max(d["n"], 1)
        d.update(total_time=self.format_interval(total_time) + " in total")
        return d

for i in TqdmExtraFormat(
      range(9), ascii=" .oO0",
      bar_format="{total_time}: {percentage:.0f}%|{bar}{r_bar}"):
    if i == 4:
        break

For example, there is format_dict that allows you to define parameters which will be used in bar_format. In above example, you can show an addition info: total_time with the extended subclass.

[martindurant]

Would it be appropriate to provide an example snippet of usage in the prose documentation where we already mention tqdm? Alternatively, you could include the exact class above in the code with docstrings (linked in the API page) explaining that this is a way to customise the progressbar functionality.

[skshetry]

I have updated the docstring with an example.