Add Meta-World, l2l.optim, l2l.vision.benchmarks.

Added

• New example: Meta-World example with MAML-TRPO with it's own env wrapper. (@Kostis-S-Z)
• l2l.vision.benchmarks interface.
• Differentiable optimization utilities in l2l.optim. (including l2l.optim.LearnableOptimizer for meta-descent)
• General gradient-based meta-learning wrapper in l2l.algorithms.GBML.
• Various nn.Modules in l2l.nn.
• l2l.update_module as a more general alternative to l2l.algorithms.maml_update.

Fixed

• clone_module supports non-Module objects.
• VGG flowers now relies on tarfile.open() instead of tarfile.TarFile().