Fix EValue construction from a smart pointer. (#5357)

Summary:
Pull Request resolved: #5357

The move constructor invoked by `*this =` expression tries to `destroy()` the `this` first, but it's not yet initialized.

For example, if the tag is randomly `Tag::ListTensor`, `destroy()` will try to iterate over and destroy a list of tensors. But since the memory is uninitialized, it will typically crash or corrupt memory.

Reviewed By: dbort

Differential Revision: D62658327

fbshipit-source-id: a12cb48f4bc3384a80534a453909e010c9549850

Author: shoumikhin

runtime/core/evalue.h

@@ -248,7 +248,9 @@ struct EValue {
           decltype(*std::forward<T>(value)),
           EValue>::value>::type* = 0) {
     ET_CHECK_MSG(value != nullptr, "Pointer is null.");
-    *this = EValue(*std::forward<T>(value));
+    // Note that this ctor does not initialize this->tag directly; it is set by
+    // moving in the new value.
+    moveFrom(*std::forward<T>(value));
   }
 
   // Delete constructor for raw pointers to ensure they cannot be used.