add doc for tensorboard (tusen-ai#221)

Author: RogerChern

doc/TENSORBOARD.md

@@ -0,0 +1,59 @@
+## Introduction
+
+### Setup tensorboard
+```bash
+pip3 install mxboard tensorboard --user
+```
+
+### Modify your config
+You need to import the `SummaryWriter` and pass it to your metric in the config.
+
+```python
+from mxboard import SummaryWriter
+
+# modify the logdir as you like
+sw = SummaryWriter(logdir="./tflogs", flush_secs=5)  
+
+rpn_acc_metric = metric.AccWithIgnore(
+    name="RpnAcc",
+    output_names=["rpn_cls_loss_output", "rpn_cls_label_blockgrad_output"],
+    label_names=[],
+    summary=sw
+)
+rpn_l1_metric = metric.L1(
+    name="RpnL1",
+    output_names=["rpn_reg_loss_output", "rpn_cls_label_blockgrad_output"],
+    label_names=[],
+    summary=sw
+)
+box_acc_metric = metric.AccWithIgnore(
+    name="RcnnAcc",
+    output_names=["bbox_cls_loss_output", "bbox_label_blockgrad_output"],
+    label_names=[],
+    summary=sw
+)
+box_l1_metric = metric.L1(
+    name="RcnnL1",
+    output_names=["bbox_reg_loss_output", "bbox_label_blockgrad_output"],
+    label_names=[],
+    summary=sw
+)
+mask_cls_metric = SigmoidCELossMetric(
+    name="MaskCE",
+    output_names=["mask_loss_output"],
+    label_names=[],
+    summary=sw
+)
+```
+
+
+### Launch tensorborad on the shell
+```bash
+# you can specify the logdir in your 
+tensorboard --logdir tflogs > /dev/null 2>&1  &
+
+python detection_train --config path/to/your/config.py
+```
+
+Now open the web browser you can see the training curve like
+![training_curve](image/tensorboard_screenshot.png)