[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/NestedUnionTest.py

@@ -0,0 +1,133 @@
+# automatically generated by the FlatBuffers compiler, do not modify
+
+# namespace: NestedUnion
+
+import flatbuffers
+from flatbuffers.compat import import_numpy
+np = import_numpy()
+
+class NestedUnionTest(object):
+    __slots__ = ['_tab']
+
+    @classmethod
+    def GetRootAs(cls, buf, offset=0):
+        n = flatbuffers.encode.Get(flatbuffers.packer.uoffset, buf, offset)
+        x = NestedUnionTest()
+        x.Init(buf, n + offset)
+        return x
+
+    @classmethod
+    def GetRootAsNestedUnionTest(cls, buf, offset=0):
+        """This method is deprecated. Please switch to GetRootAs."""
+        return cls.GetRootAs(buf, offset)
+    # NestedUnionTest
+    def Init(self, buf, pos):
+        self._tab = flatbuffers.table.Table(buf, pos)
+
+    # NestedUnionTest
+    def Name(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(4))
+        if o != 0:
+            return self._tab.String(o + self._tab.Pos)
+        return None
+
+    # NestedUnionTest
+    def DataType(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(6))
+        if o != 0:
+            return self._tab.Get(flatbuffers.number_types.Uint8Flags, o + self._tab.Pos)
+        return 0
+
+    # NestedUnionTest
+    def Data(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(8))
+        if o != 0:
+            from flatbuffers.table import Table
+            obj = Table(bytearray(), 0)
+            self._tab.Union(obj, o)
+            return obj
+        return None
+
+    # NestedUnionTest
+    def Id(self):
+        o = flatbuffers.number_types.UOffsetTFlags.py_type(self._tab.Offset(10))
+        if o != 0:
+            return self._tab.Get(flatbuffers.number_types.Int16Flags, o + self._tab.Pos)
+        return 0
+
+def NestedUnionTestStart(builder): builder.StartObject(4)
+def Start(builder):
+    return NestedUnionTestStart(builder)
+def NestedUnionTestAddName(builder, name): builder.PrependUOffsetTRelativeSlot(0, flatbuffers.number_types.UOffsetTFlags.py_type(name), 0)
+def AddName(builder, name):
+    return NestedUnionTestAddName(builder, name)
+def NestedUnionTestAddDataType(builder, dataType): builder.PrependUint8Slot(1, dataType, 0)
+def AddDataType(builder, dataType):
+    return NestedUnionTestAddDataType(builder, dataType)
+def NestedUnionTestAddData(builder, data): builder.PrependUOffsetTRelativeSlot(2, flatbuffers.number_types.UOffsetTFlags.py_type(data), 0)
+def AddData(builder, data):
+    return NestedUnionTestAddData(builder, data)
+def NestedUnionTestAddId(builder, id): builder.PrependInt16Slot(3, id, 0)
+def AddId(builder, id):
+    return NestedUnionTestAddId(builder, id)
+def NestedUnionTestEnd(builder): return builder.EndObject()
+def End(builder):
+    return NestedUnionTestEnd(builder)
+import MyGame.Example.NestedUnion.Any
+import MyGame.Example.NestedUnion.TestSimpleTableWithEnum
+import MyGame.Example.NestedUnion.Vec3
+try:
+    from typing import Union
+except:
+    pass
+
+class NestedUnionTestT(object):
+
+    # NestedUnionTestT
+    def __init__(self):
+        self.name = None  # type: str
+        self.dataType = 0  # type: int
+        self.data = None  # type: Union[None, MyGame.Example.NestedUnion.Vec3.Vec3T, MyGame.Example.NestedUnion.TestSimpleTableWithEnum.TestSimpleTableWithEnumT]
+        self.id = 0  # type: int
+
+    @classmethod
+    def InitFromBuf(cls, buf, pos):
+        nestedUnionTest = NestedUnionTest()
+        nestedUnionTest.Init(buf, pos)
+        return cls.InitFromObj(nestedUnionTest)
+
+    @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, nestedUnionTest):
+        x = NestedUnionTestT()
+        x._UnPack(nestedUnionTest)
+        return x
+
+    # NestedUnionTestT
+    def _UnPack(self, nestedUnionTest):
+        if nestedUnionTest is None:
+            return
+        self.name = nestedUnionTest.Name()
+        self.dataType = nestedUnionTest.DataType()
+        self.data = MyGame.Example.NestedUnion.Any.AnyCreator(self.dataType, nestedUnionTest.Data())
+        self.id = nestedUnionTest.Id()
+
+    # NestedUnionTestT
+    def Pack(self, builder):
+        if self.name is not None:
+            name = builder.CreateString(self.name)
+        if self.data is not None:
+            data = self.data.Pack(builder)
+        NestedUnionTestStart(builder)
+        if self.name is not None:
+            NestedUnionTestAddName(builder, name)
+        NestedUnionTestAddDataType(builder, self.dataType)
+        if self.data is not None:
+            NestedUnionTestAddData(builder, data)
+        NestedUnionTestAddId(builder, self.id)
+        nestedUnionTest = NestedUnionTestEnd(builder)
+        return nestedUnionTest