add numeric type hierarchy and issubdtype as well as a set_dtype meth… (#427)

* add numeric type hierarchy and issubdtype as well as a set_dtype method to nn.Module with predicate

numeric type hierarchy and issubtype is compatible to the [numpy hierarchy](220f0ab2c5/numpy/_core/numerictypes.py (L42)).

Closes #285.

* nits in docs

* unify type category checking

* nits in docs

* nits in docs

* more docs nits

* fix callable type

---------

Co-authored-by: Awni Hannun <awni@apple.com>

docs/src/python/array.rst

@@ -19,7 +19,6 @@ Array
     array.ndim
     array.shape
     array.size
-    Dtype
     array.abs
     array.all
     array.any
@@ -32,7 +31,6 @@ Array
     array.cumsum
     array.diag
     array.diagonal
-    array.dtype
     array.exp
     array.flatten
     array.log