Fix missing export_for_training import in bundled io tutorial (#5949)

Summary:
Add missing `export_for_training` import in bundledio tutorial.
Pull Request resolved: #5949

Reviewed By: Olivia-liu, dvorjackz

Differential Revision: D63951201

fbshipit-source-id: 42648d6d3e09edfd77714cf4a474d64dcd6c5e07

Author: tarun292

docs/source/bundled-io.md

@@ -313,7 +313,7 @@ from executorch.exir import to_edge
 from executorch.devtools import BundledProgram
 
 from executorch.devtools.bundled_program.config import MethodTestCase, MethodTestSuite
-from torch.export import export
+from torch.export import export, export_for_training
 
 
 class Module(torch.nn.Module):
@@ -449,7 +449,7 @@ from executorch.exir import to_edge
 from executorch.devtools import BundledProgram
 
 from executorch.devtools.bundled_program.config import MethodTestCase, MethodTestSuite
-from torch.export import export
+from torch.export import export, export_for_training
 
 
 class Module(torch.nn.Module):