Update nnet.txt

Fixes mismatch between the formula in implemented function in the docs

Partially factorize GpuKernelBase.get_params() and configure it to use ParamsType.

remove theano/compat/six.py

Allow c_support_code() to support list of strings and update ParamsType

Closes: #5761

Update ParamsType.c_code_cache_version().

(small update) Add restrictions to warning messages condition in GpuDnnConvGradW

 - Show warnings only if floatX==float32.
 - Show warnings only for cuDNN prior to v6.

add gpuarray to python2 buildbot

Mention duplicated values in warning messages.

Adapt local_gpu_elemwise optimization of new gpuarray back-end to avoid number of inputs overflow with Elemwise<add,mul>. The current optimization was already splitting the input, but it was not using the method split_huge_add_or_mul because of new gpuarray lifter signature (see comment 
https://github.com/Theano/Theano/pull/5852#discussion_r114145523)

The unit test for large number of inputs was invalid because it was testing the old back-end (theano.sandbox.cuda). It is now adapted to gpuarray lifter optimization function. The number of settings tested is reduced to lower the computation time while still making sure we test at least one case with no number of inputs overflow and at least one case with number of inputs overflow.

split_huge_add_or_mul() is made more general so it can be used if any case like Elemwise<add,mul> occurs elsewhere.

BUG: fix bytes/string concat error, closes #5887

Add default GpuKernelBase.get_params() that retrieves GPU context
from first node input type, and then factorize get_params() for all
sub-classes that retrieve context by this way.

Method get_params() must be overridden for all subclasses that get
context by different way.

Extend GpuKernelBase interface to allow sub-classes
to use parameters types other than gpu_context_type
(especially ParamsType).

Remove bug in optimization

 - return a list of strings for c_support_code().
 - add utility functions (to be used in other PRs).
 - Fix typos.

(help avoir duplicated support codes in final module code).

[BUG, CRASH] Fixes in DebugMode for GPU

The addition gave non-sensical results.

It also fixes a race condition.

CPU wraps around, but GPU returns max_int16, so DebugMode was
complaining.