Repository: Unbabel/COMET
Issue: Progress bar should go to stderr not stdout
Root cause: Validation Gap

## 🐛 Bug

I'm running
```bash
for i in *.output.txt; do
 comet-score -s src.txt -r newstest2021.en-de.ref.ref-A.de -t $i >$i.comet
done
```
and the progress bar is going into $i.comet.  

### To Reproduce 

```bash
comet-score -s src.txt -r ref.txt -t hyp.txt >output
less output
```

### Expected behaviour

It would be better to:
1. Send progress to stderr instead of stdout.  
2. Sense if the progress bar is going to a terminal and suppress if not.  

### Screenshots
`less output` does this
```
^MPredicting: 0it [00:00, ?it/s]^MPredicting:   1%|          | 1/126 [00:01<02:30,  1.20s/it]^MPredicting:   2%|▏         | 2/126 [00:01<01:30,  1.37it/s]^MPredicting:   2%|▏         | 3/126 [00:01<01:13,  1.67it/s]^MPredicting:   3%|▎         | 4/126 [00:02<01:07,  1.81it/s]^MPredicting:   4%|▍         | 5/126 [00:02<01:02,  1.94it/s]^MPredicting:   5%|▍         | 6/126 [00:02<00:58,  2.05it/s]^MPredicting:   6%|▌         | 7/126 [00:03<00:55,  2.14it/s]^MPredicting:   6%|▋         | 8/126 [00:03<00:52,  2.26it/s]^MPredicting:   7%|▋         | 9/126 [00:03<00:50,  2.30it/s]^MPredicting:   8%|▊         | 10/126 [00:04<00:48,  2.40it/s]^MPredicting:   9%|▊         | 11/126 [00:04<00:46,  2.45it/s]^MPredicting:  10%|▉         | 12/126 [00:04<00:46,  2.47it/s]^MPredicting:  10%|█         | 13/126 [00:05<00:46,  2.45it/s]^MPredicting:  11%|█         | 14/126 [00:05<00:45,  2.48it/s]^MPredicting:  12%|█▏        | 15/126 [00:05<00:43,  2.53it/s]^MPredicting:  13%|█▎        | 16/126 [00:06<00:43,  2.54it/s]^MPredicting:  13%|█▎        | 17/126 [00:06<00:42,  2.58it/s]^MPredicting:  14%|█▍        | 18/126 [00:06<00:41,  2.62it/s]^MPredicting:  15%|█▌        | 19/126 [00:07<00:40,  2.65it/s]^MPredicting:  16%|█▌        | 20/126 [00:07<00:39,  2.66it/s]^MPredicting:  17%|█▋        | 21/126 [00:07<00:39,  2.66it/s]^MPredicting:  17%|█▋        | 22/126 [00:08<00:39,  2.63it/s]^MPredicting:  18%|█▊        | 23/126 [00:08<00:38,  2.64it/s]^MPredicting:  19%|█▉        | 24/126 [00:09<00:38,  2.65it/s]^MPredicting:  20%|█▉        | 25/126 [00:09<00:37,  2.68it/s]^MPredicting:  21%|██        | 26/126 [00:09<00:36,  2.71it/s]^MPredicting:  21%|██▏       | 27/126 [00:09<00:36,  2.74it/s]^MPredicting:  22%|██▏       | 28/126 [00:10<00:35,  2.76it/s]^MPredicting:  23%|██▎       | 29/126 [00:10<00:35,  2.77it/s]^MPredicting:  24%|██▍       | 30/126 [00:10<00:34,  2.76it/s]^MPredicting:  25%|██▍       | 31/126 [00:11<00:34,  2.76it/s]^MPredicting:  25%|██▌       | 32/126 [00:11<00:34,  2.76it/s]^MPredicting:  26%|██▌       | 33/126 [00:11<00:33,  2.77it/s]^MPredicting:  27%|██▋       | 34/126 [00:12<00:32,  2.80it/s]^MPredicting:  28%|██▊       | 35/126 [00:12<00:32,  2.83it/s]^MPredicting:  29%|██▊       | 36/126 [00:12<00:31,  2.85it/s]^MPredicting:  29%|██▉       | 37/126 [00:12<00:31,  2.86it/s]^MPredicting:  30%|███       | 38/126 [00:13<00:30,  2.87it/s]^MPredicting:  31%|███       | 39/126 [00:13<00:30,  2.88it/s]^MPredicting:  32%|███▏      | 40/126 [00:13<00:29,  2.89it/s]^MPredicting:  33%|███▎      | 41/126 [00:14<00:29,  2.89it/s]^MPredicting:  33%|███▎      | 42/126 [00:14<00:28,  2.90it/s]^MPredicting:  34%|███▍      | 43/126 [00:14<00:28,  2.92it/s]^MPredicting:  35%|███▍      | 44/126 [00:14<00:27,  2.94it/s]^MPredicting:  36%|███▌      | 45/126 [00:15<00:27,  2.93it/s]^MPredicting:  37%|███▋      | 46/126 [00:15<00:27,  2.93it/s]^MPredicting:  37%|███▋      | 47/126 [00:16<00:27,  2.89it/s]^MPredicting:  38%|███▊      | 48/126 [00:16<00:27,  2.88it/s]^MPredicting:  39%|███▉      | 49/126 [00:16<00:26,  2.91it/s]^MPredicting:  40%|███▉      | 50/126 [00:17<00:25,  2.93it/s]^MPredicting:  40%|████      | 51/126 [00:17<00:25,  2.93it/s]^MPredicting:  41%|████▏     | 52/126 [00:17<00:25,  2.94it/s]^MPredicting:  42%|████▏     | 53/126 [00:18<00:24,  2.94it/s]^MPredicting:  43%|████▎     | 54/126 [00:18<00:24,  2.95it/s]^MPredicting:  44%|████▎     | 55/126 [00:18<00:24,  2.92it/s]^MPredicting:  44%|████▍     | 56/126 [00:19<00:24,  2.91it/s]^MPredicting:  45%|████▌     | 57/126 [00:19<00:23,  2.91it/s]^MPredicting:  46%|████▌     | 58/126 [00:19<00:23,  2.91it/s]^MPredicting:  47%|████▋     | 59/126 [00:20<00:23,  2.91it/s]^MPredicting:  48%|████▊     | 60/126 [00:20<00:22,  2.90it/s]^MPredicting:  48%|████▊     | 61/126 [00:20<00:22,  2.91it/s]^MPredicting:  49%|████▉     | 62/126 [00:21<00:22,  2.91it/s]^MPredicting:  50%|█████     | 63/126 [00:21<00:21,  2.91it/s]^MPredicting:  51%|█████     | 64/126 [00:22<00:21,  2.89it/s]^MPredicting:  52%|█████▏    | 65/126 [00:22<00:21,  2.90it/s]^MPredicting:  52%|█████▏    | 66/126 [00:22<00:20,  2.88it/s]^MPredicting:  53%|█████▎    | 67/126 [00:23<00:20,  2.85it/s]^MPredicting:  54%|█████▍    | 68/126 [00:23<00:20,  2.84it/s]^MPredicting:  55%|█████▍    | 69/126 [00:24<00:20,  2.85it/s]^MPredicting:  56%|█████▌    | 70/126 [00:24<00:19,  2.86it/s]^MPredicting:  56%|█████▋    | 71/126 [00:24<00:19,  2.87it/s]^MPredicting:  57%|█████▋    | 72/126 [00:25<00:18,  2.87it/s]^MPredicting:  58%|█████▊    | 73/126 [00:25<00:18,  2.87it/s]^MPredicting:  59%|█████▊    | 74/126 [00:25<00:18,  2.87it/s]^MPredicting:  60%|█████▉    | 75/126 [00:25<00:17,  2.89it/s]^MPredicting:  60%|██████    | 76/126 [00:26<00:17,  2.90it/s]^MPredicting:  61%|██████    | 77/126 [00:26<00:16,  2.90it/s]^MPredicting:  62%|██████▏   | 78/126 [00:26<00:16,  2.91it/s]^MPredicting:  63%|██████▎   | 79/126 [00:27<00:16,  2.91it/s]^MPredicting:  63%|██████▎   | 80/126 [00:27<00:15,  2.91it/s]^MPredicting:  64%|██████▍   | 81/126 [00:27<00:15,  2.90it/s]^MPredicting:  65%|██████▌   | 82/126 [00:28<00:15,  2.90it/s]^MPredicting:  66%|██████▌   | 83/126 [00:28<00:14,  2.92it/s]^MPredicting:  67%|██████▋   | 84/126 [00:28<00:14,  2.93it/s]^MPredicting:  67%|██████▋   | 85/126 [00:28<00:13,  2.93it/s]^MPredicting:  68%|██████▊   | 86/126 [00:29<00:13,  2.93it/s]^MPredicting:  69%|██████▉   | 87/126 [00:29<00:13,  2.91it/s]^MPredicting:  70%|██████▉   | 88/126 [00:30<00:13,  2.91it/s]^MPredicting:  71%|███████   | 89/126 [00:30<00:12,  2.90it/s]^MPredicting:  71%|███████▏  | 90/126 [00:30<00:12,  2.90it/s]^MPredicting:  72%|███████▏  | 91/126 [00:31<00:12,  2.91it/s]^MPredicting:  73%|███████▎  | 92/126 [00:31<00:11,  2.90it/s]^MPredicting:  74%|███████▍  | 93/126 [00:32<00:11,  2.90it/s]^MPredicting:  75%|███████▍  | 94/126 [00:32<00:11,  2.89it/s]^MPredicting:  75%|███████▌  | 95/126 [00:32<00:10,  2.89it/s]^MPredicting:  76%|███████▌  | 96/126 [00:33<00:10,  2.88it/s]^MPredicting:  77%|███████▋  | 97/126 [00:33<00:10,  2.88it/s]^MPredicting:  78%|███████▊  | 98/126 [00:34<00:09,  2.88it/s]^MPredicting:  79%|███████▊  | 99/126 [00:34<00:09,  2.87it/s]^MPredicting:  79%|███████▉  | 100/126 [00:34<00:09,  2.88it/s]^MPredicting:  80%|████████  | 101/126 [00:35<00:08,  2.88it/s]^MPredicting:  81%|████████  | 102/126 [00:35<00:08,  2.88it/s]^MPredicting:  82%|████████▏ | 103/126 [00:35<00:07,  2.88it/s]^MPredicting:  83%|████████▎ | 104/126 [00:36<00:07,  2.88it/s]^MPredicting:  83%|████████▎ | 105/126 [00:36<00:07,  2.88it/s]^MPredicting:  84%|████████▍ | 106/126 [00:36<00:06,  2.87it/s]^MPredicting:  85%|████████▍ | 107/126 [00:37<00:06,  2.87it/s]^MPredicting:  86%|████████▌ | 108/126 [00:37<00:06,  2.87it/s]^MPredicting:  87%|████████▋ | 109/126 [00:38<00:05,  2.86it/s]^MPredicting:  87%|████████▋ | 110/126 [00:38<00:05,  2.86it/s]^MPredicting:  88%|████████▊ | 111/126 [00:38<00:05,  2.86it/s]^MPredicting:  89%|████████▉ | 112/126 [00:39<00:04,  2.86it/s]^MPredicting:  90%|████████▉ | 113/126 [00:39<00:04,  2.85it/s]^MPredicting:  90%|█████████ | 114/126 [00:39<00:04,  2.85it/s]^MPredicting:  91%|█████████▏| 115/126 [00:40<00:03,  2.85it/s]^MPredicting:  92%|█████████▏| 116/126 [00:40<00:03,  2.85it/s]^MPredicting:  93%|█████████▎| 117/126 [00:41<00:03,  2.85it/s]^MPredicting:  94%|█████████▎| 118/126 [00:41<00:02,  2.85it/s]^MPredicting:  94%|█████████▍| 119/126 [00:41<00:02,  2.84it/s]^MPredicting:  95%|█████████▌| 120/126 [00:42<00:02,  2.81it/s]^MPredicting:  96%|█████████▌| 121/126 [00:43<00:01,  2.80it/s]^MPredicting:  97%|█████████▋| 122/126 [00:43<00:01,  2.79it/s]^MPredicting:  98%|█████████▊| 123/126 [00:44<00:01,  2.79it/s]^MPredicting:  98%|█████████▊| 124/126 [00:44<00:00,  2.80it/s]^MPredicting:  99%|█████████▉| 125/126 [00:44<00:00,  2.80it/s]^MPredicting: 100%|██████████| 126/126 [00:44<00:00,  2.81it/s]^MPredicting: 100%|██████████| 126/126 [00:45<00:00,  2.79it/s]
```

### Environment
OS: Ubuntu 20.04 x86_64
Packaging pip
Version pip install unbabel-comet==1.0.0rc2

### Additional context


Thanks for reporting this issue. I changed the Lightning Progress bar to write to stderr. 

I tested both commands:
```bash
comet-score -s src.de -t hyp.en -r ref.en
```
and 
```bash
comet-score -s src.de -t hyp.en -r ref.en > output
```

in the first command, you get the progress bar and scores on the terminal. On the second command, you get the progress bar on the terminal but the scores go to the output file.

If you have any more requests please just ask and we can include them on the new release which should be "official" after this year's Metrics shared task.