Tensorboard path generalisation (#804)

* Allow experiment versions to be overridden by passing a string value.
Allow experiment names to be empty, in which case no per-experiment subdirectory will be created and checkpoints will be saved in the directory given by the save_dir parameter.

* Document tensorboard api changes

* Review comment fixes plus fixed test failure for minimum requirements build

* More format fixes from review

tests/test_logging.py

@@ -294,6 +294,19 @@ def test_tensorboard_manual_versioning(tmpdir):
     assert logger.version == 1
 
 
+def test_tensorboard_named_version(tmpdir):
+    """Verify that manual versioning works for string versions, e.g. '2020-02-05-162402' """
+
+    tmpdir.mkdir("tb_versioning")
+    expected_version = "2020-02-05-162402"
+
+    logger = TensorBoardLogger(save_dir=tmpdir, name="tb_versioning", version=expected_version)
+    logger.log_hyperparams({"a": 1, "b": 2})  # Force data to be written
+
+    assert logger.version == expected_version
+    # Could also test existence of the directory but this fails in the "minimum requirements" test setup
+
+
 @pytest.mark.parametrize("step_idx", [10, None])
 def test_tensorboard_log_metrics(tmpdir, step_idx):
     logger = TensorBoardLogger(tmpdir)