Given a tensor like

```
a = 1 1 1
1 1 1
1 1 1
2 1 1
2 1 1
```

I would like to obtain all rows starting with 2, i.e. I would like to obtain

```
b = 2 1 1
2 1 1
```

Indexing using `a[a[:, 0]==2]`

doesn’t seem to work and `index_select`

requires indices to be known and so does `gather`

.