Fix compile warnings (#1762)

* Fix comparison between signed and unsigned integer expressions
* Remove unused variable

torchaudio/csrc/pybind/sox/effects.cpp

@@ -54,7 +54,7 @@ auto apply_effects_fileobj(
     // end up retrieving the static variable defined in `_torchaudio`, which is
     // not correct.
     const auto bufsiz = get_buffer_size();
-    const size_t kDefaultCapacityInBytes = 256;
+    const int64_t kDefaultCapacityInBytes = 256;
     return (bufsiz > kDefaultCapacityInBytes) ? bufsiz
                                               : kDefaultCapacityInBytes;
   }();

torchaudio/csrc/pybind/sox/io.cpp

@@ -43,7 +43,7 @@ auto get_info_fileobj(py::object fileobj, c10::optional<std::string> format)
     // end up retrieving the static variable defined in `_torchaudio`, which is
     // not correct.
     const auto bufsiz = get_buffer_size();
-    const size_t kDefaultCapacityInBytes = 4096;
+    const int64_t kDefaultCapacityInBytes = 4096;
     return (bufsiz > kDefaultCapacityInBytes) ? bufsiz
                                               : kDefaultCapacityInBytes;
   }();