[Python] Fixed the issue with nested unions relying on InitFromBuf. (#7576)

* feat: Fixed the issue with nested unions relying on InitFromBuf.
Problem: Issue #7569
Nested Unions were broken with the introduction of parsing buffers with an initial encoding offset.

Fix:
Revert the InitFromBuf method to the previous version and introduction of InitFromPackedBuf that allows
users to read types from packed buffers applying the offset automatically.

Test:
Added in TestNestedUnionTables to test the encoding and decoding ability using a nested table with a
union field.

* fix: Uncommented generate code command

tests/MyGame/Example/NestedUnion/TestSimpleTableWithEnum.py

@@ -0,0 +1,78 @@
+# automatically generated by the FlatBuffers compiler, do not modify
+
+# namespace: NestedUnion
+
+import flatbuffers
+from flatbuffers.compat import import_numpy
+np = import_numpy()
+
+class TestSimpleTableWithEnum(object):
+    __slots__ = ['_tab']
+
+    @classmethod
+    def GetRootAs(cls, buf, offset=0):
+        n = flatbuffers.encode.Get(flatbuffers.packer.uoffset, buf, offset)
+        x = TestSimpleTableWithEnum()
+        x.Init(buf, n + offset)
+        return x
+
+    @classmethod
+    def GetRootAsTestSimpleTableWithEnum(cls, buf, offset=0):
+        """This method is deprecated. Please switch to GetRootAs."""
+        return cls.GetRootAs(buf, offset)
+    # TestSimpleTableWithEnum
+    def Init(self, buf, pos):
+        self._tab = flatbuffers.table.Table(buf, pos)
+
+    # TestSimpleTableWithEnum
+    def Color(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(4))
+        if o != 0:
+            return self._tab.Get(flatbuffers.number_types.Uint8Flags, o + self._tab.Pos)
+        return 2
+
+def TestSimpleTableWithEnumStart(builder): builder.StartObject(1)
+def Start(builder):
+    return TestSimpleTableWithEnumStart(builder)
+def TestSimpleTableWithEnumAddColor(builder, color): builder.PrependUint8Slot(0, color, 2)
+def AddColor(builder, color):
+    return TestSimpleTableWithEnumAddColor(builder, color)
+def TestSimpleTableWithEnumEnd(builder): return builder.EndObject()
+def End(builder):
+    return TestSimpleTableWithEnumEnd(builder)
+
+class TestSimpleTableWithEnumT(object):
+
+    # TestSimpleTableWithEnumT
+    def __init__(self):
+        self.color = 2  # type: int
+
+    @classmethod
+    def InitFromBuf(cls, buf, pos):
+        testSimpleTableWithEnum = TestSimpleTableWithEnum()
+        testSimpleTableWithEnum.Init(buf, pos)
+        return cls.InitFromObj(testSimpleTableWithEnum)
+
+    @classmethod
+    def InitFromPackedBuf(cls, buf, pos=0):
+        n = flatbuffers.encode.Get(flatbuffers.packer.uoffset, buf, pos)
+        return cls.InitFromBuf(buf, pos+n)
+
+    @classmethod
+    def InitFromObj(cls, testSimpleTableWithEnum):
+        x = TestSimpleTableWithEnumT()
+        x._UnPack(testSimpleTableWithEnum)
+        return x
+
+    # TestSimpleTableWithEnumT
+    def _UnPack(self, testSimpleTableWithEnum):
+        if testSimpleTableWithEnum is None:
+            return
+        self.color = testSimpleTableWithEnum.Color()
+
+    # TestSimpleTableWithEnumT
+    def Pack(self, builder):
+        TestSimpleTableWithEnumStart(builder)
+        TestSimpleTableWithEnumAddColor(builder, self.color)
+        testSimpleTableWithEnum = TestSimpleTableWithEnumEnd(builder)
+        return testSimpleTableWithEnum