[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/nested_union_test.fbs

@@ -0,0 +1,36 @@
+namespace MyGame.Example.NestedUnion;
+
+/// Composite components of Monster color.
+enum Color:ubyte (bit_flags) {
+  Red = 0, // color Red = (1u << 0)
+  /// \brief color Green
+  /// Green is bit_flag with value (1u << 1)
+  Green,
+  /// \brief color Blue (1u << 3)
+  Blue = 3,
+}
+
+table TestSimpleTableWithEnum (csharp_partial, private) {
+  color: Color = Green;
+}
+
+struct Test { a:short; b:byte; }
+
+table Vec3 {
+  x:double;
+  y:double;
+  z:double;
+  test1:double;
+  test2:Color;
+  test3:Test;
+}
+
+union Any { Vec3, TestSimpleTableWithEnum }
+
+table NestedUnionTest {
+  name:string;
+  data:Any;
+  id:short;
+}
+
+root_type NestedUnionTest;