desc is an attribute whose value is a string that describes a status or indicator in the training process. In this case, train_bar.desc is a string that records information about each training epoch, including the current number of epochs, the total number of epochs and the training loss. Specifically, this string uses format(), the string formatting method in Python, with the values of the variables epoch, epochs, and loss inserted into the string at their appropriate positions by padding the {} placeholders and arguments in the format() method. The resulting string is of the form "train epoch[1/10] loss:0.123".
