Rewrite long nested `Sequential` types using `Sequential{N}` typealiases

[dan-zheng]

Sequential{N} typealiases were added in tensorflow/swift-apis#934:

import TensorFlow
// Before:
typealias Model = Sequential<Dense<Float>, Sequential<Dense<Float>, Dense<Float>>>
// After:
typealias Model = Sequential3<Dense<Float>, Dense<Float>, Dense<Float>>

The tuple-like typealiases are easier to use. Long nested Sequential types should be rewritten to use them:

swift-models/pix2pix/Discriminator.swift

Line 18 in b0cc1c4

var module: Sequential<Sequential<Conv2D<Float>, Sequential<Function<Tensor<Float>, Tensor<Float>>, Sequential<Conv2D<Float>, Sequential<BatchNorm<Float>, Sequential<Function<Tensor<Float>, Tensor<Float>>, Sequential<Conv2D<Float>, Sequential<BatchNorm<Float>, Function<Tensor<Float>, Tensor<Float>>>>>>>>>, Sequential<ConvLayer, Sequential<BatchNorm<Float>, Sequential<Function<Tensor<Float>, Tensor<Float>>, ConvLayer>>>>

[andrewasche]

Can I do this?

[dan-zheng]

Sure!

I don't know if tensorflow/swift-apis#934 has propagated to "nightly" (six hourly) toolchains yet, I think it should have.