Change micro_speech example (#2249)

@tensorflow/micro 

Create Python examples that use Signal Ops Library.
Create Python examples for micro_speech model prediction.
Update README documentation.
Update images.
Add additional speech sample files (silence, noise) to testdata directory.
Add audio preprocessing models to models  directory.
Update Bazel BUILD files.
Update Makefiles
Add micro_speech to project generation examples for bluepill
Disable micro_speech_test for Hexagon (b/302404477)

bug=fixes  #2248

python/tflite_micro/signal/BUILD

@@ -33,7 +33,7 @@ py_library(
         "ops/__init__.py",
     ],
     srcs_version = "PY3",
-    visibility = ["//python/tflite_micro/signal/utils:__subpackages__"],
+    visibility = ["//visibility:public"],
     deps = [
         ":delay_op",
         ":energy_op",

python/tflite_micro/signal/utils/BUILD

@@ -37,6 +37,7 @@ pybind_extension(
 py_library(
     name = "util",
     srcs = ["util.py"],
+    visibility = ["//visibility:public"],
     deps = [
         requirement("tensorflow-cpu"),
         "//python/tflite_micro:runtime",
@@ -71,4 +72,5 @@ py_test(
         "nomsan",
         "noubsan",
     ],
+    visibility = ["//visibility:public"],
 )

python/tflite_micro/signal/utils/wide_dynamic_func_lut_wrapper.cc

@@ -42,7 +42,8 @@ int16_t PcanGainLookupFunction(const float strength, const float offset,
 
 py::list WideDynamicFuncLut(float strength, float offset, int input_bits,
                             int gain_bits) {
-  int16_t* gain_lut = new int16_t[kWideDynamicFunctionLUTSize];
+  int16_t gain_lut_storage[kWideDynamicFunctionLUTSize];
+  int16_t* gain_lut = gain_lut_storage;
 
   gain_lut[0] =
       PcanGainLookupFunction(strength, offset, gain_bits, input_bits, 0);
@@ -52,7 +53,7 @@ py::list WideDynamicFuncLut(float strength, float offset, int input_bits,
   // loop below a lot simpler.
   gain_lut -= 6;
 
-  for (int interval = 2; interval <= kWideDynamicFunctionBits; ++interval) {
+  for (size_t interval = 2; interval <= kWideDynamicFunctionBits; ++interval) {
     const uint32_t x0 = static_cast<uint32_t>(1) << (interval - 1);
     const uint32_t x1 = x0 + (x0 >> 1);
     const uint32_t x2 =
@@ -80,10 +81,9 @@ py::list WideDynamicFuncLut(float strength, float offset, int input_bits,
   gain_lut += 6;
 
   py::list lut_list = py::list();
-  for (int i = 0; i < kWideDynamicFunctionLUTSize; i++) {
+  for (size_t i = 0; i < kWideDynamicFunctionLUTSize; i++) {
     lut_list.append(gain_lut[i]);
   }
-  free(gain_lut);
 
   return lut_list;
 }