Add LayerNorm and GroupNorm

[lkhphuc]

Add LayerNorm and GroupNorm, also move rename BatchNorm file to Norm.

Unrelated question: I saw in the Treex's module, you defined module's properties as both class variables (dataclass style) and as init's parameters? What is the reason behind this? I though the point of dataclass-like attribute is to reduce the boilerplate in __init__?

[cgarciae]

Hey @lkhphuc, thanks a lot! I'll give it a review soon.

Meanwhile I'll answer your question:

Unrelated question: I saw in the Treex's module, you defined module's properties as both class variables (dataclass style) and as init's parameters? What is the reason behind this? I though the point of dataclass-like attribute is to reduce the boilerplate in init?

There are 2 reasons I think, the first one is that using class variables is a good way to provide metadata for fields regardless if its a dataclass or not. The second one is that if you do it like this you immediately have support for dataclasses and non-dataclasses which is ideal because you want to have flexibility, Treex (or rather treeo) supports both.

[cgarciae]

@lkhphuc reviewed it, I have no comments, great job! 🎉