gradient should use initializer::NONE

[kparichay]

Gradients currently use initializer::ZEROS which initializes the gradient for the first iteration. However, gradients are not initialized to zero for the next iterations and the layer is expected to initialize the gradients to zero if expected.

To maintain this consistency, gradients should be initialized for initializer::NONE.

[taos-ci]

:octocat: cibot: Thank you for posting issue #1805. The person in charge will reply soon.