If you were using some flavour of Linux you could easily monitor GPU memory allocations with nvidia-smi command. Probably mac has something similar. For instance on my PC I see the following:
$ nvidia-smi 
Tue Nov 20 00:13:02 2018       
+-----------------------------------------------------------------------------+
| NVIDIA-SMI 410.66       Driver Version: 410.66       CUDA Version: 10.0     |
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|===============================+======================+======================|
|   0  GeForce GTX 1080    Off  | 00000000:03:00.0  On |                  N/A |
|  0%   47C    P2    44W / 200W |   1418MiB /  8118MiB |      0%      Default |
+-------------------------------+----------------------+----------------------+
                                                                               
+-----------------------------------------------------------------------------+
| Processes:                                                       GPU Memory |
|  GPU       PID   Type   Process name                             Usage      |
|=============================================================================|
|    0       804      G   /usr/lib/Xorg                                398MiB |
|    0      2462      G   ...uest-channel-token=17047571084676707762   141MiB |
|    0      5242      C   /usr/bin/python                              875MiB |
+-----------------------------------------------------------------------------+