* The PybindAdaptors.h file has been evolving across different sub-projects (npcomp, circt) and has been successfully used for out of tree python API interop/extensions and defining custom types.
* Since sparse_tensor.encoding is the first in-tree custom attribute we are supporting, it seemed like the right time to upstream this header and use it to define the attribute in a way that we can support for both in-tree and out-of-tree use (prior, I had not wanted to upstream dead code which was not used in-tree).
* Adapted the circt version of `mlir_type_subclass`, also providing an `mlir_attribute_subclass`. As we get a bit of mileage on this, I would like to transition the builtin types/attributes to this mechanism and delete the old in-tree only `PyConcreteType` and `PyConcreteAttribute` template helpers (which cannot work reliably out of tree as they depend on internals).
* Added support for defaulting the MlirContext if none is passed so that we can support the same idioms as in-tree versions.
There is quite a bit going on here and I can split it up if needed, but would prefer to keep the first use and the header together so sending out in one patch.
Differential Revision: https://reviews.llvm.org/D102144
Sean Silva