Added custom model example

[jafioti]

I think it's important to show how to build a custom module that implements custom functionality so I took the mnist example and created a custom MLP struct. Obviously this is a trivial scenario, but it demonstrates how to use the traits to implement a new module for both single and batch forward functions.

[coreylowman]

Good idea! I'm thinking this example could just be the custom model definition and then in main call forward with single & batched tensor. Thoughts? Thinking that including all the mnist stuff just detracts from the example!

[coreylowman]

Yeah I definitely want to merge this in, but let's reduce it a bit to make it as clear as possible! Let me know if you aren't planning on updating and I can take this over.

[jafioti]

Sounds good, I'll reduce it down to just the core example later today.

[jafioti]

Alright I simplified the example to just a single forward and batched forward. Let me know what you think.

[coreylowman]

Looks great! Thanks for the contribution 🎉