fixed_vector (#2372)

* 1) class Symbol; 2) return const Shape& from Blob::shape() (#2360)

* 1) class Symbol; 2) return const Shape& from Blob::shape()

* const Symbol<Shape> => const Symbol<Shape>&

* refactor Symbol<T>

* CachedCaller

* define Symbol<T>::ptr_ as std::atomic<const T*>

* replace unique_lock with lock_guard

* refactor Blob::shape()

* refactor Blob::shape_sym

* backup

* refactor cached caller

* op_infer_cache

* ThreadLocalCachedCall

* clang-format

* refactor Blob::shape()

* CheckNumAxesIdenticalAndAssign

* refactor symbol

* rename: FindOrInsertPtr => GetOrCreatePtr

* 1) LeftOnesExtendedAssign; 2) LeftOnesStrippedAssign

* fix a stupid bug in Symbol::operator!=

* fixed_vector

* rename: ToDimVec => ToDimVector