[FBcode->GH] Add back cifar_root

[fmassa]

This is needed for some internal tests, adding this back for now

[pmeier]

@fmassa if someone depends on the old fake data generation, they could use this instead:

import contextlib

from test_datasets import CIFAR10TestCase, CIFAR100
from common_utils import get_tmp_dir


@contextlib.contextmanager
def dataset_root_from_test_case(cls, config=None):
    if config is None:
        cls._populate_private_class_attributes()
        config = cls._DEFAULT_CONFIG

    with get_tmp_dir() as root:
        test_case = cls()
        test_case.inject_fake_data(root, config)

        yield root


@contextlib.contextmanager
def cifar_root(version):
    with dataset_root_from_test_case(CIFAR10TestCase if version == "CIFAR10" else CIFAR100) as root:
        yield root