Remove Auto-Flattening of Chains

[avik-pal]

See #681. Code becomes type unstable for Zygote if the NamedTuple has $\geq 32$ fields. We were flattening to help the zygote, but that seems to cause problems for larger models.

This is of-course extremely breaking, and will break all of the Boltz models, but I think we take that hit early on.