- Gradient strategies to filter out non-observing circuits associated with identity term.

- For two-sided gradients (central and parameter shift), the identity term has no effect. For single-sided ones (forward and backward), add the identity coefficient in the shifted enery value.

- Fixed an oversight in setting optional 'step' param. It was only set in the ctor, but not reset during initialization. Hence, it may carry the value set by a previous run.

- Added a set of unit test cases covering 'shots' mode

Signed-off-by: Thien Nguyen <thien.md.nguyen@gmail.com>