live: Add dvclive logger and DVCLIVE_LOGLEVEL.

[daavoo]

Closes #206

• ❗ I have followed the Contributing to DVCLive guide.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here.

[pared]

LGTM. Tested on a actual repo using matplotlib too. Seems like setting the DEBUG level will result in logs from this lib too. Not sure how to approach that. Probably we should not propagate it down to other used libraries.

[daavoo]

LGTM. Tested on a actual repo using matplotlib too. Seems like setting the DEBUG level will result in logs from this lib too. Not sure how to approach that. Probably we should not propagate it down to other used libraries.

Updated P.R. It should no longer affect other libs, could you confirm?

[pared]

I can confirm that now I see only dvclive logs for DEBUG option.

INFO:dvclive:Report will be saved at /Users/pawelredzynski/development/projects/flexible_plots_pg/training_logs/report.html
Model created
Prepared the datasets, train size: '2500', test size: '10000'
epoch:   0%|                                                                                                        | 0/4 [00:00<?, ?it/s]
DEBUG:dvclive:Logged train_loss: 0.9999628314748407
DEBUG:dvclive:Logged test_loss: 0.5283907478212551
DEBUG:dvclive:Logged train_accuracy: 0.751953125
DEBUG:dvclive:Logged test_accuracy: 0.8277269108280255
DEBUG:dvclive:Step: 1
train loss: '1.0000', test_loss: '0.5284, train_acc: '0.7520', test_acc: '0.8277':  25%|█████               | 1/4 [00:15<00:47, 15.84s/it]
DEBUG:dvclive:Logged train_loss: 0.30712428838014605
DEBUG:dvclive:Logged test_loss: 0.2543744519827472
DEBUG:dvclive:Logged train_accuracy: 0.920703125
DEBUG:dvclive:Logged test_accuracy: 0.9219745222929936
DEBUG:dvclive:Step: 2
train loss: '0.3071', test_loss: '0.2544, train_acc: '0.9207', test_acc: '0.9220':  50%|██████████          | 2/4 [00:21<00:19,  9.61s/it]
DEBUG:dvclive:Logged train_loss: 0.18037323066964744
DEBUG:dvclive:Logged test_loss: 0.22544675505465003
DEBUG:dvclive:Logged train_accuracy: 0.954296875
DEBUG:dvclive:Logged test_accuracy: 0.9304339171974523
DEBUG:dvclive:Step: 3
train loss: '0.1804', test_loss: '0.2254, train_acc: '0.9543', test_acc: '0.9304':  75%|███████████████     | 3/4 [00:26<00:07,  7.62s/it]
DEBUG:dvclive:Logged train_loss: 0.12118110828123463
DEBUG:dvclive:Logged test_loss: 0.21492478192137304
DEBUG:dvclive:Logged train_accuracy: 0.9734375
DEBUG:dvclive:Logged test_accuracy: 0.9357085987261147
DEBUG:dvclive:Step: 4
train loss: '0.1212', test_loss: '0.2149, train_acc: '0.9734', test_acc: '0.9357': 100%|████████████████████| 4/4 [00:31<00:00,  7.91s/it]