Stannum

This release only contains minor fixes that does not change the behaviors of `stannum`

Super excited to announce our new `stannum`!

Here are two major changes to `Tube`:
* To avoid any potential confusion, negative indices in `Tube` for don't-care(any) dim and matching dim and `None` for batching dim are deprecated. Instead, use explicit `stannum.AnyDim`, `stannum.MatchDim(dim_id)` and `stannum.BatchDim` in 0.9 and onward.
* `DimensionCalculator` is added to enable dynamic dimension calculation in `Tube`, which will be super useful in cases like convolution.

The documentation is updated as well. Check it out!

Since last release:
* A bug has been fix. The bug appears when after forward computation, if we update kernel extra args using set_kernel_extra_args (once or multiple times), backward computation is messed up due to inconsistent kernel inputs during forward and backward passes.
* The APIs of `Tin` and `EmptyTin` have been changed: the constructors need `auto_clear_grad` specified, which is a reminder to users that gradients of fields must be taken care of carefully so to not have incorrect gradients after multiple runs of `Tin` or `EmptyTin` layers.

Nothing big has change in the code base of `stannum`, but since Taichi developers have delivered a long waited performance improvement, I want to urge everyone using `stannum` to update Taichi in use to `1.1.3`. And some kind warning and documentation are added to help `stannum` users to understand this important upstream update.

Got prepared for Taichi v1.1.0, which is expected to contain a lot of refactoring and API changes.