Map and apply with multiprocessing

# Context


we want to

```python
da.apply(lambda x : x, num_worker=4 )
```


This will be different from doing 

```python
for doc in da:
    doc = f(doc)

map(f,a)
```


Is that we will leverage multiprocessing. (Need to be extra careful and add good test to check that nothing is going wrong)
+ benchmarking 


We need to choose between Map or Apply but we need only of them.


