Merge pull request !2875 from wangqiuliang/add-grad-all-in-pynative

add comment for control_depend

pynative.

To handle 'break' and 'continue' statement, a loop context is pushed
to a stack before we parse the loop body, and pop it after body parsed.
When a 'break', 'continue' statement is encountered, we retrieve current
loop contex from the stack, and let the current block jump to the end
block or header block;

For 'break' statement, we added an extra 'end_block' follow the 'after_block',
because 'after_block' is called from a ContionalJump in 'header_block', it can
not be set as jump target from other place. to support 'break', we let loop
body jump to the 'end_block' at the 'break' point. and 'after_block'
maybe a good place to handle loop 'else' clause in the future.

Handle 'pass' is simple, just bypass it when doing parse.

Signed-off-by: leonwanghui <leon.wanghui@huawei.com>