Updates _login to make it usable in Api._load_api_key and fixes a bug in _load_api_key.

Pulls out the _noop, _offline and wandb.run checks into the higher level wandb.login() (which is also called by the wandb login CLI). The _noop and _offline checks are already present in wandb.init() and the wandb.run check is not relevant there. The fact that Api._load_api_key doesn't work if the identity_token_file setting is set is the same as it was before.

_login is called by login() (which is called by the wandb login CLI), wandb.init(), Sweeps, and now Api._load_api_key(). The changes above shouldn't matter for Sweeps.

I recently refactored _load_api_key() to use the new auth methods in PR #10893 but in doing so introduced a subtle bug: if Api._load_api_key prompts for a key, that key is not saved in the session, and later calls to Api() will prompt again. I will add an auth method for this soon, but due to other obstacles, using _login is the cleaner refactoring path.