Need warning for cross validation

[mortonjt]

There is a scenario where if you specify your samples for training and testing and they get filtered out, you may get this error. This basically says that your testing samples were filtered out. A better message is needed

(from tensorflow.python.ops.random_ops) is deprecated and will be removed in a future version.
...skipping...
 accuracy/Log (defined at /lib/python3.6/site-packages/mmvec/multimodal.py:159)

Input Source operations connected to node accuracy/multinomial/Multinomial:
 accuracy/Log (defined at /lib/python3.6/site-packages/mmvec/multimodal.py:159)

Original stack trace for 'accuracy/multinomial/Multinomial':
  File "/bin/mmvec", line 225, in <module>
    mmvec()
  File "/lib/python3.6/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/lib/python3.6/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/lib/python3.6/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/lib/python3.6/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/lib/python3.6/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/bin/mmvec", line 168, in paired_omics
    test_microbes_coo, test_metabolites_df.values)
  File "/lib/python3.6/site-packages/mmvec/multimodal.py", line 160, in __call__
    self.cv_size)
  File "/lib/python3.6/site-packages/tensorflow/python/util/deprecation.py", line 324, in new_func
    return func(*args, **kwargs)
  File "/lib/python3.6/site-packages/tensorflow/python/ops/random_ops.py", line 357, in multinomial
    return multinomial_categorical_impl(logits, num_samples, output_dtype, seed)
  File "/lib/python3.6/site-packages/tensorflow/python/ops/random_ops.py", line 393, in multinomial_categorical_impl
    logits, num_samples, seed=seed1, seed2=seed2, output_dtype=dtype)
  File "/lib/python3.6/site-packages/tensorflow/python/ops/gen_random_ops.py", line 85, in multinomial
    seed2=seed2, output_dtype=output_dtype, name=name)
  File "/lib/python3.6/site-packages/tensorflow/python/framework/op_def_library.py", line 788, in _apply_op_helper
    op_def=op_def)
  File "/lib/python3.6/site-packages/tensorflow/python/util/deprecation.py", line 507, in new_func
    return func(*args, **kwargs)
  File "/lib/python3.6/site-packages/tensorflow/python/framework/ops.py", line 3616, in create_op
    op_def=op_def)
  File "/lib/python3.6/site-packages/tensorflow/python/framework/ops.py", line 2005, in __init__
    self._traceback = tf_stack.extract_stack()