Add support for nested generics (#1104)

* Add support for nested generics

* Allow instantiation of unparameterized generics

* Add better more partial instantiation tests

* Add changes

* Add docs

docs/examples/models_generics_nested.py

@@ -0,0 +1,21 @@
+from typing import Generic, TypeVar
+
+from pydantic import ValidationError
+from pydantic.generics import GenericModel
+
+T = TypeVar('T')
+
+class InnerT(GenericModel, Generic[T]):
+    inner: T
+
+class OuterT(GenericModel, Generic[T]):
+    outer: T
+    nested: InnerT[T]
+
+nested = InnerT[int](inner=1)
+print(OuterT[int](outer=1, nested=nested))
+try:
+    nested = InnerT[str](inner='a')
+    print(OuterT[int](outer='a', nested=nested))
+except ValidationError as e:
+    print(e)