docstr: ...Test that original coeffs are correctly retrieved when some
  realizations are empty
docstr_tok: ["...","Test","that","original","coeffs","are","correctly","retrieved","when","some","realizations","are","empty"]
code: def test_sparse(self):
      """...Test that original coeffs are correctly retrieved when some
      realizations are empty
      """
      baseline, adjacency, events = self.simulate_sparse_realization()
  
      learner = HawkesSumExpKern(self.decays, verbose=False)
      learner.fit(events)
  
      np.testing.assert_array_almost_equal(
          learner.baseline, baseline, decimal=1
      )
      np.testing.assert_array_almost_equal(
          learner.adjacency, adjacency, decimal=1
      )
  
imports: import unittest
  import numpy
  from tick.base.inference import InferenceTest
  from tick.hawkes import SimuHawkesSumExpKernels, SimuHawkesMulti
  from tick.hawkes.inference import HawkesSumExpKern
  from tick.prox import ProxNuclear
  from tick.prox import ProxPositive, ProxL1, ProxL2Sq, ProxElasticNet
  from tick.solver import AGD, GD, BFGS, SGD, SVRG
label_desc: Use image node_id 4 for calling the Test obj's underlying member method code with example usage: obj.test_sparse() without return types