* Update requirements.txt
I found that if we would like to calculate FLOPS in this project, we must install thop.
but there's no thop package inside the requirements.txt
https://github.com/ultralytics/yolov3/blob/master/utils/torch_utils.py#L108
* Update requirements.txt
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>
In a clean environment running training fails if tensorboard is not installed e.g.
```
Traceback (most recent call last):
File "/home/josh/miniconda3/envs/ultralytics/lib/python3.7/site-packages/torch/utils/tensorboard/__init__.py", line 2, in <module>
from tensorboard.summary.writer.record_writer import RecordWriter # noqa F401
ModuleNotFoundError: No module named 'tensorboard'
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "train.py", line 6, in <module>
from torch.utils.tensorboard import SummaryWriter
File "/home/josh/miniconda3/envs/ultralytics/lib/python3.7/site-packages/torch/utils/tensorboard/__init__.py", line 4, in <module>
raise ImportError('TensorBoard logging requires TensorBoard with Python summary writer installed. '
ImportError: TensorBoard logging requires TensorBoard with Python summary writer installed. This should be available in 1.14 or above.
```
