[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/Test.py

@@ -0,0 +1,66 @@
+# automatically generated by the FlatBuffers compiler, do not modify
+
+# namespace: NestedUnion
+
+import flatbuffers
+from flatbuffers.compat import import_numpy
+np = import_numpy()
+
+class Test(object):
+    __slots__ = ['_tab']
+
+    @classmethod
+    def SizeOf(cls):
+        return 4
+
+    # Test
+    def Init(self, buf, pos):
+        self._tab = flatbuffers.table.Table(buf, pos)
+
+    # Test
+    def A(self): return self._tab.Get(flatbuffers.number_types.Int16Flags, self._tab.Pos + flatbuffers.number_types.UOffsetTFlags.py_type(0))
+    # Test
+    def B(self): return self._tab.Get(flatbuffers.number_types.Int8Flags, self._tab.Pos + flatbuffers.number_types.UOffsetTFlags.py_type(2))
+
+def CreateTest(builder, a, b):
+    builder.Prep(2, 4)
+    builder.Pad(1)
+    builder.PrependInt8(b)
+    builder.PrependInt16(a)
+    return builder.Offset()
+
+
+class TestT(object):
+
+    # TestT
+    def __init__(self):
+        self.a = 0  # type: int
+        self.b = 0  # type: int
+
+    @classmethod
+    def InitFromBuf(cls, buf, pos):
+        test = Test()
+        test.Init(buf, pos)
+        return cls.InitFromObj(test)
+
+    @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, test):
+        x = TestT()
+        x._UnPack(test)
+        return x
+
+    # TestT
+    def _UnPack(self, test):
+        if test is None:
+            return
+        self.a = test.A()
+        self.b = test.B()
+
+    # TestT
+    def Pack(self, builder):
+        return CreateTest(builder, self.a, self.b)