Crop `x` to `size` pixels. `row_pct`,`col_pct` select focal point of crop.

def _crop_default(x, size, row_pct:uniform=0.5, col_pct:uniform=0.5):
    "Crop `x` to `size` pixels. `row_pct`,`col_pct` select focal point of crop."
    rows,cols = tis2hw(size)
    row_pct,col_pct = _minus_epsilon(row_pct,col_pct)
    row = int((x.size(1)-rows+1) * row_pct)
    col = int((x.size(2)-cols+1) * col_pct)
    return x[:, row:row+rows, col:col+cols].contiguous()