fix Parameter.__repr__ behaviour within tf.function/graph mode (#1247)
fix Parameter.__repr__ behaviour within tf.function/graph mode (#1247)

Parameter.__repr__ called .numpy() on the constrained and unconstrained representations - this throws an error when in graph mode (tf.function-wrapped), which can lead to errors-in-exception-handling, so this PR makes use of tf.eagerly_executing() to check whether we are in graph mode, and if so, only print shapes and dtype, not values.
GPflow/GPflow