code: def _causal_mask(length):
      # Not all backends define jnp.tril. However, using np.tril is inefficient
      # in that it creates a large global constant. TODO(kitaev): try to find an
      # alternative that works across all backends.
      if fastmath.is_backend(fastmath.Backend.JAX):
          return jnp.tril(
              jnp.ones((1, length, length), dtype=np.bool_), k=0
          )
      else:
          return np.tril(
              np.ones((1, length, length), dtype=np.bool_), k=0
          )
  
imports: import numpy
  from trax import fastmath
  from trax.fastmath import numpy
  from trax.layers import base
  from trax.layers import combinators
  from trax.layers import convolution
  from trax.layers import core
  from trax.layers import initializers
  from trax.layers.assert_shape import assert_shape
  from trax.layers.base import Fn
  from trax.layers.research import sparsity
label_desc: Use image node_id 8 for calling a global function with example usage: _causal_mask(length) and returns: jnp, np