[RFC] Lift freqs_cis as an input of models

[XilunWu]

Stack from ghstack (oldest at bottom):

• #1901
• #1897
• #1884
• #1883
• -> #1882

freqs_cis is sensitive to the sequence order. CP load balancing will shuffle the samples, so each batch will have different orders. As a result, we will have to lift these order senstive buffer to the inputs and broadcast them along the batch dimension so that PP will correctly shard freqs_cis without messing up the correctness.

Pull-Request-resolved: https://github.com/pytorch/torchtitan/pull/1797