Backend-specific detectors inherit incorrect name in meta

[ascillitoe]

Detectors with a backend, such as MMDDrift inherit their backend-specific subclass names in their self.meta dict i.e, MMDDriftTF. The top-level detector name i.e. MMDDrift should instead be stored. See https://github.com/SeldonIO/alibi-detect/pull/497#discussion_r880276630.