basic support for logging warnings

[amandlek]

• adding functions log_warning and flush_warnings to utils/log_utils.py, allowing us to log warnings (in yellow text by default) at the start of training, and cache them so that they are displayed once again right before training starts, so that they appear all together in a convenient location that's easy to check while debugging
• to leverage, call log_warning with the warning message, and optionally specify the text color (default is "yellow"), and whether you want to print the warning immediately (in addition to printing later when flush_warnings is called)