#7838 implement batch barrier feature, and we also need to implement merging SelectedRows in server site dynamically, just because:

1. After the distributed transpiler, a SelectedRows variable block maybe contains an empty Tensor, and we don't need to send the empty Tensor in SendOp.
2. For the server site, we can not use an Op to merge the received SelectedRows, because the Input size should be a definite value, but the number of received SelectedRows variable is different between each batch.

So I think we also need to implement the following feature:

1. Send Op don't need to send the variable which was not initialized.
2. Merge SelectedRows in server site.