Add an API to change the datatype of an initialized module

We should have some API to change the dtype of a module after it has been initialized. 
Without such an API, the user has to propagate dtype through the module at construction time, which is cumbersome. 

Possible APIs:
```py
module = MyModule()

module.to(tp.float16)

module.cast(tp.float16)

module = tp.cast(module, tp.float16)
```