[bugfix] resolve undefine reference error

This commit fixes the undefined reference to nntrainer::Quantizer::custom_quantizer

**Self-evaluation:**
1. Build test: [X]Passed [ ]Failed [ ]Skipped
2. Run test:   [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: Donghyeon Jeong <dhyeon.jeong@samsung.com>

Author: djeong20

nntrainer/tensor/quantizer.h

@@ -45,7 +45,7 @@ enum class QScheme : uint8_t {
  */
 class Quantizer {
 private:
-  static std::unordered_map<QScheme, Quantizer *>
+  static inline std::unordered_map<QScheme, Quantizer *>
     custom_quantizers; /** Hash table that holds empty instances of the custom
                           quantizers */