[RFC] Merge train_with_engine and train_with_trainer

[FrankLeeeee]

Describe the feature

In most examples, there are two files, namely train with engine and trainer. The code is highly redundant in these two files and we should just merge them into one file.
We can add an additional flag to let the user to choose to run with either engine or trainer. As engine provides better portability from user code to colossalai-style code, we should keep engine as the default.