[caffe2] update db::Transaction::Put() to accept the value by rvalue reference (#60208)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60208
Update the DB APIs so that `db::Transaction::Put()` accepts the value by
rvalue reference. This allows DB implementations to write data asynchronously
without being forced to make an additional copy of the data in memory.
`Put()` implementations can now use the string move constructor or assignment
operator to get the string data and continue performing the write
asynchronously after returning from `Put()`.
Note that I chose to entirely replace the existing `Put()`, removing the
ability for callers to call `Put()` with a `const std::string&` argument for
the value, rather than simply adding another overloaded version of `Put()`.
This was done because in practice there were no call sites using `Put()` that
cannot move in their data. Eliminating the `const std::string&` API entirely
simplifies the DB implementations: DBs that wish do support move semantics do
not have to implement both the move and the copy versions of `Put()`.
Test Plan:
Searched through fbcode to try and make sure I found all `db::Transaction`
subclasses, and will check sandcastle results to help confirm.
Ran the modelstore checkpointing unit tests.
Differential Revision: D29204425
fbshipit-source-id: 28be6646e92e5df71954d4bb3dc0c8add30ed041
