Replace ndarray with dfdx.

[a-rustacean]

dfdx stands out with its advanced type safety and feature-rich capabilities when contrasted with ndarray. It encompasses crucial components for building neural networks, including versatile Tensors, comprehensive neural network modules, and compatibility with both Cpu and Cuda devices. However, it's important to note that the initiation of work on this project may be delayed, as dfdx is currently in its pre-alpha stage. Nevertheless, we eagerly anticipate its stable release.