Support offline learning of the multiple trails

```python

import brainpy as bp

i = bp.nn.Input(1)
r = bp.nn.Reservoir(100)
o = bp.nn.Dense(1)

model = i >> r >> o

model.initialize(num_batch=1)
```

Currently, we only support offline learning with batch size is 1. This cannot satisfy the needs in the real data. We should extend it to learn with multiple trials. 
