Handle comingling of mutable and constant weights in flattensor .ptd

[JacobSzwejbka]

🚀 The feature, motivation and pitch

These will need to be serialized in 2 separate segments. Currently we just serialize all external tensors in the same segment which will cause memory issues down the line if we start having configurations that want to have both in the same .ptd

Alternatives

No response

Additional context

No response

RFC (Optional)

No response

cc @angelayi