Add Kendall's Tau-c, Spearman's Rank, Pearson's Correlation metrics (#2741)

* renaming kendalls_tau to streaming_correlations

* Fix keras import

* Fix test function naming

* Fixing commit history to improve review experience

Author: nicolaspi

tensorflow_addons/metrics/__init__.py

@@ -31,4 +31,9 @@
 from tensorflow_addons.metrics.r_square import RSquare
 from tensorflow_addons.metrics.geometric_mean import GeometricMean
 from tensorflow_addons.metrics.harmonic_mean import HarmonicMean
-from tensorflow_addons.metrics.kendalls_tau import KendallsTau
+from tensorflow_addons.metrics.streaming_correlations import (
+    KendallsTauB,
+    KendallsTauC,
+    PearsonsCorrelation,
+    SpearmansRank,
+)