Torchlayers

This is a minor change before first release targeted towards wider audience.

Improvements

- `torchlayers.Infer` class is now `torchlayers.infer` function and __shouldn't be used as decorator__ as it destroys `torch.build` due to `pickle` error (see [this issue](https://stackoverflow.com/questions/52185507/pickle-and-decorated-classes-picklingerror-not-the-same-object) for example).
Now, it should be used as a function as described in [`README.md`](https://github.com/szymonmaszke/torchlayers/blob/master/README.mdcustom-modules-with-shape-inference-capabilities).
- Removed support of `Python 3.6` as `torchlayers` relies on `3.7` module wide `__getattr__` and `__dir__`

This is initial release of `torchlayers`, project should be considered as being in alpha stage.

See [README.md](https://github.com/szymonmaszke/torchlayers/blob/master/README.md) to get a grip on what this library does.

See [documentation](https://szymonmaszke.github.io/torchlayers/) for in-depth description of provided layers & functionalities.