Define correct toString method for all existing layers

[zaleslaw]

A part of layers, like the Dense layer, have a broken toString method which could not print the state of the layer before building its part of TF Graph.

We suggest improving library maintainers' experience during debugging and defining toString method for all layers which will be independent of the stage of model initialization and compilation.

[zaleslaw]

For example I see in debugger Method threw 'kotlin.UninitializedPropertyAccessException' exception. Cannot evaluate org.jetbrains.kotlinx.dl.api.core.layer.convolutional.Conv1D.toString() before weights and variables initialization and the same for all layers with the variables