Automatically determine parquet workload record_count

[daverigby]

For parquet file workloads we can determine the number of records from the actual parquet files, it is not necessary to manually specify it as we currently do:

class Mnist(MnistBase):
    def __init__(self, name: str, cache_dir: str):
        super().__init__(name, "mnist", cache_dir=cache_dir)

    @property
    def record_count(self) -> int:
        return 60000

Similarly for any workload which limits the count such as *-test variants, we also know the limit:

class MnistTest(MnistBase):
    """Reduced, "test" variant of mnist; with 1% of the full dataset (600
    passages and 20 queries)."""

    def __init__(self, name: str, cache_dir: str):
        super().__init__(name, "mnist", cache_dir=cache_dir, limit=600, query_limit=20)

    @property
    def record_count(self) -> int:
        return 600