Allow to deactivate GPU memory logging in Trainer (#190)

* Allow to deactivate GPU memory logging in Trainer

Adds the flag `log_gpu_memory` to Trainer to deactivate logging of GPU
memory utilization. On some servers logging the GPU memory usage can
significantly slow down training.

* Update Logging.md

* Update trainer.py

docs/Trainer/Logging.md

@@ -14,6 +14,14 @@ Every k batches lightning will make an entry in the metrics log
 ``` {.python}
 # DEFAULT (ie: save a .csv log file every 10 batches)
 trainer = Trainer(add_log_row_interval=10)
+```   
+
+---
+#### Log metric row every k batches 
+Logs GPU memory when metrics are logged.   
+``` {.python}
+# DEFAULT
+trainer = Trainer(log_gpu_memory=False)
 ```
 
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