Fix Fashion MNIST autoencoder example

[nskins]

The Fashion MNIST autoencoder example throws the same error as the examples in #437. This PR fixes the autoencoder by updating the dependencies and fixing breaking changes.

Summary of changes:

• Update dependencies to latest versions
• Add :batch placeholder for dynamic batch size
• Update deprecated Nx function to_batched_list/3 to to_batched/3
• Use Enum.fetch!/2 since hd/1 does not work on a stream
• Remove unused alias for Axon.Loop.State
• Fix spelling of MNIST in module name

[seanmor5]

Thanks! @nskins