Add conditional random fields (#314)

* Port CRF from tf.contrib to tfa.text

Author: Squadrick

tensorflow_addons/text/BUILD

@@ -6,6 +6,7 @@ py_library(
     name = "text",
     srcs = ([
         "__init__.py",
+        "crf.py",
         "skip_gram_ops.py",
     ]),
     data = [
@@ -15,6 +16,19 @@ py_library(
     srcs_version = "PY2AND3",
 )
 
+py_test(
+    name = "crf_test",
+    size = "small",
+    srcs = [
+        "crf_test.py",
+    ],
+    main = "crf_test.py",
+    srcs_version = "PY2AND3",
+    deps = [
+        ":text",
+    ],
+)
+
 py_test(
     name = "skip_gram_ops_test",
     size = "small",

tensorflow_addons/text/README.md

@@ -4,6 +4,7 @@
 | Submodule  |  Maintainers  | Contact Info   |
 |:---------- |:----------- |:------------- |
 | skip_gram_ops |  |  |
+| crf | Dheeraj R. Reddy | [email protected] |
 
 ## Components 
 | Submodule  | Text Processing Function |  Reference  |

tensorflow_addons/text/__init__.py

@@ -17,6 +17,19 @@
 from __future__ import division
 from __future__ import print_function
 
+# Conditional Random Field
+from tensorflow_addons.text.crf import crf_binary_score
+from tensorflow_addons.text.crf import crf_decode
+from tensorflow_addons.text.crf import crf_decode_backward
+from tensorflow_addons.text.crf import crf_decode_forward
+from tensorflow_addons.text.crf import crf_forward
+from tensorflow_addons.text.crf import crf_log_likelihood
+from tensorflow_addons.text.crf import crf_log_norm
+from tensorflow_addons.text.crf import crf_multitag_sequence_score
+from tensorflow_addons.text.crf import crf_sequence_score
+from tensorflow_addons.text.crf import crf_unary_score
+from tensorflow_addons.text.crf import viterbi_decode
+
 # Skip Gram Sampling
 from tensorflow_addons.text.skip_gram_ops import skip_gram_sample
-from tensorflow_addons.text.skip_gram_ops import skip_gram_sample_with_text_vocab
+from tensorflow_addons.text.skip_gram_ops import skip_gram_sample_with_text_vocab