[2.5] Remove test/predict in lightning fl example

[YuanTingHsieh]

Issue

When using the external process (SubprocessLauncher)

(1) In the last round the CJ will consider the job finish right after "fit" is called, thus the predict/test stage of external program will be killed in the middle

(2) the standalone lightning training script (examples/hello-world/ml-to-fl/pt/src/cifar10_lightning_original.py) is not working because of import (from src.net import Net)

Description

• Remove the test/predict stage as it is not required
• Move the Net definition inside the lit_net.py file so import will not be an issue when running standalone lightning training

Types of changes

• Non-breaking change (fix or new feature that would not break existing functionality).
• Breaking change (fix or new feature that would cause existing functionality to change).
• New tests added to cover the changes.
• Quick tests passed locally by running ./runtest.sh.
• In-line docstrings updated.
• Documentation updated.

[nvidianz]

/build