Remove inf's in TruncatedNormal log_prob & sample (#1492)

[nikmich1]

This commit fixes #1492 by removing inf return values in log_prob and sample method of the TruncatedNormal distribution, when it is truncated in the tail of the distribution.

Changes made to remove inf's in log_prob method of TruncatedNormal distribution:

• Added log_cdf method to the Normal distribution
  • Calls the corresponding JAX implementation of the logarithmic cdf
• Modified log_prob calculation for the truncated Normal
  • New method is more stable in tail of distribution, since it uses the new log_cdf, if it is available
  • Uses logsumexp function to subtract required log_cdf values more numerically stable
  • If log_cdf method is not available, it falls back to original implementation with cdf

Changes made to remove inf's in sample method of TruncatedNormal distribution:

• Clamped input to inverse cdf icdf to open interval (0,1) using the clamp_probs utility

Tests added:

• Test implementation of log_cdf method by comparing to JAX implementation and to cdf method (test_normal_log_cdf)
• Test log_prob implementation by comparing to JAX (test_truncated_normal_log_prob_in_tail)
• Test sample method in tail of distribution to check, if any inf's are returned (test_sample_truncated_normal_in_tail)

[fehiepsi]

Looks great to me, thanks @nikmich1!