[Feature] Support custom placement strategy from user

Users' network setup might not be a full network mesh. Thus, we should allow users to connect in the following way:

Node: A, B, C, D
A<->B with RDMA
C<->D with RDMA

thus A and C should be in the same group(either trainer or inference).


We should allow something such as:

```
- Trainer
   - IP0
   - IP1
- Inference
   - IP2
   - IP3
```

to avoid our auto placement