More autograd helpers

[yaugenst-flex]

• Removes DeprecationWarning of DataArray.values.
• Adds smooth_min, smooth_max, and least_squares implementations to plugins.autograd.functions.
• Adds @scalar_objective decorator that extracts DataArray.data and performs some additional checks on the output data of objective functions. If an objective function is decorated with this, calling DataArray.values within the objective function is not necessary anymore. Our grad and value_and_grad implementations in plugins.autograd wrap objective functions with this decorator automatically.
• Some internal refactoring of the pad function.
• Some test refactoring to reduce the no. of function calls for grad checks.

[tylerflex]

Looks great! think at some point we should include all of these in our demo notebooks. Just a few minor comments.