Remove references to `Dataset`

[texasmichelle]

Dataset is deprecated and has been replaced with the Batches API (formerly Epochs). Replace all references in this repo before Dataset is removed entirely.

See tensorflow/swift-models#495 and tensorflow/swift-models#603 for examples of this conversion.

This will resolve warnings like this:

[50/51] Compiling FastaiNotebook_04_callbacks 04_callbacks.swift
/fastai_dev/swift/FastaiNotebook_04_callbacks/Sources/FastaiNotebook_04_callbacks/04_callbacks.swift:27:25: warning: 'Dataset' is deprecated: Datasets will be removed in S4TF v0.10. Please use the new Batches API instead.
    public var innerDs: Dataset<Element>