Remove invalid chunk from introduction vignette

[rargelaguet]

The following chunk is not valid for tensorflow 2:

sess = tf$Session()
hello <- tf$constant('Hello, TensorFlow!')
sess$run(hello)

returns:

Error in py_get_attr_impl(x, name, silent) : 
  AttributeError: module 'tensorflow' has no attribute 'Session'

[chanwkimlab]

You can execute

tf <- tf$compat$v1;
tf$disable_v2_behavior()

before executing the chunk of code you mentioned to resolve the problem,