[XLA:MHLO->HLO] Allow partially-set parameter tuple sharding to exist by filling in the missing sharding elements with replicated sharding. (This is what is done for the missing shardings in the result tuple.)
[XLA:MHLO->HLO] Allow partially-set parameter tuple sharding to exist by filling in the missing sharding elements with replicated sharding. (This is what is done for the missing shardings in the result tuple.)

Before this change, if an element of a tuple parameter did not have a sharding, MHLO->HLO conversion dropped the existing annotations on the parameter. This issue caused the disappearing of the parameter sharding for a model, which then resulted in an OOM.
