[caffe2] use AddNAlreadyReserved() when serializing blobs
Summary:
Optimize the blob serialization code by using `AddNAlreadyReserved()` when
serializing tensor data, rather than making N separate `Add()` calls.
`AddNAlreadyReserved()` is a simple addition operation, while each `Add()`
call checks to see if it needs to reserve new space, and then updates the
element data, which is unnecessary in this case.
Test Plan:
This appears to improve raw serialization performance by 30 to 35% for float,
double, and int64_t types which use this function. This improvement appears
relatively consistent across large and small tensor sizes.
Differential Revision: D26617038
fbshipit-source-id: 97dedbae889d35463628f3016ac56986e685289e
