On Google Colab, trying to use FlaxBigBirdForMaskedLM shows that Flax is not installed.
from transformers import BigBirdConfig

config = BigBirdConfig(
    vocab_size=40000,
    hidden_size = 768,
    max_position_embeddings=16000,
    num_attention_heads=4,                   #6
    num_hidden_layers=4,                    #6
)

from transformers import FlaxBigBirdForMaskedLM
model = FlaxBigBirdForMaskedLM(config=config)

It may be due to some other issue, But I am consistenly getting this error across all runtimes (target runtime is TPU).
ImportError: 
FlaxBigBirdForMaskedLM requires the FLAX library but it was not found in your environment. Checkout the instructions on the
installation page: https://github.com/google/flax and follow the ones that match your environment.

Flax is indeed installed, via !pip install -q transformers[flax].
Does this seem like a genuine bug? The problem is that the backend Flax is apparently not accessible by the method, while I can easily import flax and other utilities.