fix paddle.Model en docs (PaddlePaddle#43537)

* add copy-from label for code examples, test=document_fix

* refine docs, test=document_fix

* add some output for code example, test=document_fix

* add `optional`, test=document_fix

* add missing parameters, test=document_fix

* add missing links for `ProgBarLogger` and `ModelCheckpoint`, test=document_fix

* update eval_batch example, test=document_fix

* fix typos in stack_outputs, test=document_fix

* np.random -> paddle.random, test=document_fix