chimera_graph mxn lattice is flattened

**Description**
I expected that ``chimera_graph(m)`` or ``chimera_graph(m,m)`` would have be a m-by-m lattice but instead am getting a row of length mxm.

**To Reproduce**
```
>>> c4 = dnx.chimera_graph(4)
>>> c4.nodes
NodeView((0, 4, 5, 6, 7, 1, 2, 3, 32, 36, 37, 38, 39, 33, 34, 35, 64, 68, 69, 70, 71, 65, 66, 67, 96, 100, 101, 102, 103, 97, 98, 99, 8, 12, 13, 14, 15, 9, 10, 11, 40, 44, 45, 46, 47, 41, 42, 43, 72, 76, 77, 78, 79, 73, 74, 75, 104, 108, 109, 110, 111, 105, 106, 107, 16, 20, 21, 22, 23, 17, 18, 19, 48, 52, 53, 54, 55, 49, 50, 51, 80, 84, 85, 86, 87, 81, 82, 83, 112, 116, 117, 118, 119, 113, 114, 115, 24, 28, 29, 30, 31, 25, 26, 27, 56, 60, 61, 62, 63, 57, 58, 59, 88, 92, 93, 94, 95, 89, 90, 91, 120, 124, 125, 126, 127, 121, 122, 123))
```
This gives:


**Expected behavior**
I expected:


**Environment:**
- OS: WIN10
- Python version: 3.7.0

**Additional context**
Add any other context about the problem here.