Align IterableDataset.shuffle with Dataset.shuffle
From #3444 , Dataset.shuffle can have the same API than IterableDataset.shuffle (i.e. in streaming mode).

Currently you can pass an optional seed to both if you want, BUT currently IterableDataset.shuffle always requires a buffer_size, used for approximate shuffling. I propose using a reasonable default value (maybe 1000) instead.

In this PR, I set the default `buffer_size` value to 1,000, and I reorder the `IterableDataset.shuffle` arguments to match `Dataset.shuffle`, i.e. making `seed` the first argument.