metrics: Use Array and ArrayLike types thoughout

[copybara-service]

metrics: Use Array and ArrayLike types thoughout

Currently the inputs to from_model_output are not typed. However,
these functions cannot accept arbitrary inputs, they need to be a value
convertable to a jax.Array. This change fixes this so that:

• from_model_output takes in types of Array or ArrayLike
• Removes use of jnp.array as a type as it's equivalent to Any
• Makes members of Metric classes have type Array
• Moves mask checking code into its own function

While we could make everything use Array (instead of ArrayLike),
this would break code like:

@flax.struct.dataclass
class Collection(metrics.Collection):
  train_accuracy: metrics.Accuracy
  learning_rate: metrics.LastValue.from_output("learning_rate")

Collection.gather_from_model_output(learning_rate=0.02, ...)

which seems undesirable.

Note that count and value for LastValue have type ArrayLike,
as this code needs to support passing a plain number for value or
count. Also, the base Metric.compute() method has type Any,
because some metrics return Array while others use dict[str, Array].