Fix some underlying issues with tensor core sample

[computablee]

After messing with the tensor core example some, myself and @tbennun noticed some issues with the sample. This PR fixes a few issues.

1. The storage types (e.g., Accumulator, TensorCore_*) have been added to defined_vars. In some cases not demonstrated in hgemm, the codegen generates improper passes to functions. By setting the type to DefinedType.Stream, we ensure that the WMMA fragments are always passed as an object reference.
2. The frontend is greatly redone and simplified, changing the use of @replaces to simple dace.tasklets. As with 1, certain situations not exposed with hgemm showed that memlets are not always properly generated with the frag_fill and wmma functions. Changing their implementation to dace.tasklet ensures that memlets are now properly generated in all use cases, along with simplifying their implementation to a large degree.

[tbennun]

LGTM, I would recommend to change the name Stream in DefinedTypes to Object or something that would make more sense.