Update MNIST model to adopt ParameterAggregate synthesis.
Update MNIST model to adopt ParameterAggregate synthesis.

Also:

* Since most bugs have been fixed, remove workarounds on scalar promotion, partitioning crashers and send/receive crashers.

* `Tensor.transposed` does not need to specify permutations.

* A `main()` function conveys a wrong message about the programming model. Swift supports and encourages top-level code for training scripts. `main()` has been changed to `train(_:iterationCount:)`, a function that actually has a meaning.

tensorflow/swift-models,tensorflow/swift-models,tensorflow/swift-models,tensorflow/swift-models