hparams: add Python layer for hparams and metrics (#2126)

Summary:
This change introduces `HParam`, `Metric`, and `Experiment` classes,
which represent their proto counterparts in a more Python-friendly way.
It similarly includes a `Domain` class hierarchy, which does not
correspond to a specific proto message, but rather unifies the domain
variants defined on the `HParamInfo` proto.

The design is roughly as in the original sketch of #1998.

The primary benefit of this change is that having first-class domains
enables clients to reuse the domain information for both the experiment
summary and the underlying tuning algorithm. We don’t provide a method
to do this out of the box, because we don’t actually provide any tuners
at this time, but it’s easy to write (e.g.) a `sample_uniform` function
like the one included in this commit. Then, sampling is as easy as

```python
    hparams = {h: sample_uniform(h.domain, rng) for h in HPARAMS}
```

It is also now more convenient to reference hparam values such that
static analysis can detect potential typos, because the `HParam` objects
themselves can be declared as constants and used as keys in a dict.
Writing `hparams["dropuot"]` fails at runtime, but `hparams[HP_DROPUOT]`
fails at lint time.

As a pleasant bonus, hparam definitions are now more compact, fitting
on one line instead of several. The demo code has net fewer lines.

Manual summary writer management is still required. A future change will
introduce a Keras callback to reduce this overhead.

Test Plan:
Some unit tests included, and the demo still works.

wchargin-branch: hparams-structured-api

tensorboard/plugins/hparams/BUILD

@@ -98,14 +98,14 @@ py_binary(
     srcs = ["hparams_demo.py"],
     srcs_version = "PY2AND3",
     deps = [
+        ":api",
         ":protos_all_py_pb2",
         ":summary",
         "//tensorboard:expect_absl_app_installed",
         "//tensorboard:expect_absl_flags_installed",
         "//tensorboard:expect_numpy_installed",
         "//tensorboard:expect_tensorflow_installed",
         "//tensorboard/plugins/scalar:summary",
-        "@com_google_protobuf//:protobuf_python",
         "@org_pythonhosted_six",
     ],
 )
@@ -149,6 +149,35 @@ sh_test(
     ],
 )
 
+py_library(
+    name = "api",
+    srcs = ["api.py"],
+    srcs_version = "PY2AND3",
+    visibility = [
+        "//visibility:public",
+    ],
+    deps = [
+        ":protos_all_py_pb2",
+        ":summary",
+        "@org_pythonhosted_six",
+    ],
+)
+
+py_test(
+    name = "api_test",
+    size = "small",
+    srcs = ["api_test.py"],
+    srcs_version = "PY2AND3",
+    deps = [
+        ":api",
+        ":metadata",
+        ":protos_all_py_pb2",
+        "//tensorboard:test",
+        "@com_google_protobuf//:protobuf_python",
+        "@org_pythonhosted_six",
+    ],
+)
+
 py_library(
     name = "summary",
     srcs = ["summary.py"],