Created by: lcy-seso

• After this PR https://github.com/PaddlePaddle/Paddle/pull/3085, we can retrieve parameter value and gradient matrices in Python end.

• Previously, Paddle has a show_parameter_status_period parameter to print some statistical information of value and gradient matrices of every parameter in a model. This is very helpful for debugging the model. But currently, v2 cannot support it.

• I think besides enabling the original show_parameter_status_period parameter, we can use this PR https://github.com/PaddlePaddle/Paddle/pull/3085 to add a show_parameter_status method into the current parameter class to print some statistical information such as the max absolute value of value/gradient matrix, the average absolute value of value/gradient matrix, and so on.

• Print more information rather than only a cost value is extremely helpful when training a complicated model. I often highly need this feature.