Refactor the heaviest of lifters

[talmo]

This function gives me nightmares:

https://github.com/talmolab/biogtr/blob/a47c48b84535632290ab7757d65abbfd787d69bc/biogtr/models/transformer.py#L444-L483

This is especially salient due to the fact that this is so deep in the call stack (TransformerDecoderLayer <- TransformerDecoder <- Transformer <- GlobalTrackingTransformer <- GTRRunner <- ...).

This induces so many code paths it's nearly impossible to trace:

https://github.com/talmolab/biogtr/blob/a47c48b84535632290ab7757d65abbfd787d69bc/biogtr/models/transformer.py#L457-L458

https://github.com/talmolab/biogtr/blob/a47c48b84535632290ab7757d65abbfd787d69bc/biogtr/models/transformer.py#L460

Why??? It's just for self-attention, so why not just keep it in that conditional a few lines down??

https://github.com/talmolab/biogtr/blob/a47c48b84535632290ab7757d65abbfd787d69bc/biogtr/models/transformer.py#L462-L465

And surely we can come up with better names than tgt, tgt2, tgt3??

https://github.com/talmolab/biogtr/blob/a47c48b84535632290ab7757d65abbfd787d69bc/biogtr/models/transformer.py#L467-L481

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Both here and in the upstream Transformer.call, let's fix the variable names, disambiguate "target"/tgt, memory, and canonical key/query/value nomenclature.

[aaprasad]

Done in #36