[caffe2] Explicitly define all DataTypes in python/core.py (pytorch#51768)

Summary:
Pull Request resolved: pytorch#51768

This updates python/core.py to explicitly define all of the `DataType`
values rather than dynamically defining them at runtime from the
`caffe2_pb2` values.

This allows type checkers like Pyre and Mypy to see the members of the
`DataType` class.  Otherwise the type checkers report errors such as
`"core.DataType" has no attribute "INT64"`.

This code does keep a run-time check that all of the data types defined
by `caffe2_pb2.proto` are defined correctly in this file.  This way if
someone does add a new type to `caffe2_pb2.proto` it should be very
quickly apparent that this file needs to be updated and kept in sync.
ghstack-source-id: 121936201

Test Plan:
Confirmed that various caffe2/python tests still pass.
Verified that this allows many `pyre-fixme` comments to be removed in
downstream projects, and that Pyre is still clean for these projects.

Reviewed By: jeffdunn

Differential Revision: D26271725

Pulled By: simpkins

fbshipit-source-id: f9e95795de60aba67d7d3872d0c141ed82ba8e39

Author: simpkins

caffe2/python/core.py

@@ -40,15 +40,36 @@
 
 # Bring datatype enums to the main namespace
 class DataType:
-    pass
-
-
-def _InitDataType():
+    UNDEFINED = 0
+    FLOAT = 1
+    INT32 = 2
+    BYTE = 3
+    STRING = 4
+    BOOL = 5
+    UINT8 = 6
+    INT8 = 7
+    UINT16 = 8
+    INT16 = 9
+    INT64 = 10
+    FLOAT16 = 12
+    DOUBLE = 13
+    ZERO_COLLISION_HASH = 14
+    REBATCHING_BUFFER = 15
+
+
+def _CheckDataType():
+    # Verify that the DataType values defined above match the ones defined in
+    # the caffe2.proto file
     for name, value in caffe2_pb2.TensorProto.DataType.items():
-        setattr(DataType, name, value)
+        py_value = getattr(DataType, name, None)
+        if py_value != value:
+            raise AssertionError(
+                f"DataType {name} does not match the value defined in "
+                f"caffe2.proto: {py_value} vs {value}"
+            )
 
 
-_InitDataType()
+_CheckDataType()
 
 
 def _GetRegisteredOperators():