Update tfds.load to load datasets from files without using original class.

[tfds-copybara]

Update tfds.load to load datasets from files without using original class.

Before: In tfds.load('my_dataset'), the DatasetBuilder was always created from the original dataset class (MyDataset).

After: If the version is specified, tfds.load/tfds.builder first check whether an existing version is found on disk. If found, only the files are used to restore the dataset (without ever instantiating the original generation class MyDataset).

• tfds.load('my_dataset:2.*.*') can load .../my_dataset/2.0.3/ files even if MyDataset.VERSION == '3.0.0'. This improve backward-compatibility.

• Dataset can be read even if the generation code isn't reachable anymore. In this case, tfds.load('my_dataset') will load the most recent version found on disk. So you can load a dataset generated by someone else without having to import the original dataset code. (caveat: in this case, the config name must be explicit, as TFDS can't currently infer the default config name).

Note: This require dataset generated with TFDS 4.0.0+

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