I have two tensors, one with data and one with some value I would like to threshold on, both along row `C`

:

```
a = torch.Tensor(B,C,X,Y)
b = torch.Tensor(B,C)
```

I would like to find those rows of `C`

in tensor `a`

that fulfill a criterion based on tensor `b`

. For example:

```
# Indices of b where b is zero
idx = (b==0)
```

The end result should look like:

```
bar(b)
foo(a)
a = torch.Tensor(B,C-N,X,Y)
#N is number of rows that do not match condition.
```