Fixed issues for LSTM

Fixes issues related to #167.

• Added maxElements input to Stack: @jeromeh I made some change to allow to specify maxElements also through an input. This is actually a necessity to enable static forward dims of a graph with the Stack operator, like in the example above for LSTM;

%%{init: {'flowchart': { 'curve': 'monotoneY'}, 'fontFamily': 'Verdana' } }%%
flowchart TB

LSTM_0("lstm_output<br/><sub><em>(LSTM#0)</em></sub>"):::metaCls_rootCls
Pop_0(<em>Pop#0</em>)
Stack_0(<em>Stack#0</em>)
Identity_0(<em>Identity#0</em>)
Shape_0(<em>Shape#0</em>)
LSTM_0-->|"0&rarr;0"|Stack_0
LSTM_0-->|"0&rarr;0"|output1((out#1)):::outputCls
LSTM_0-->|"1&rarr;0"|output2((out#2)):::outputCls
Pop_0-->|"0&rarr;0"|LSTM_0
Identity_0-->|"0&rarr;0"|Pop_0
Identity_0-->|"0&rarr;0"|Shape_0
Shape_0-->|"0&rarr;1"|Stack_0
input0((in#0)):::inputCls--->|"&rarr;0"|Identity_0
Stack_0--->|"0&rarr;"|output0((out#0)):::outputCls
classDef inputCls fill:#afa
classDef outputCls fill:#ffa
classDef externalCls fill:#ccc
classDef producerCls fill:#ccf
classDef genericCls fill:#f9f9ff,stroke-width:1px,stroke-dasharray: 5 5
classDef metaCls stroke-width:5px
classDef rootCls stroke:#f00
classDef producerCls_rootCls stroke:#f00,fill:#ccf
classDef genericCls_rootCls stroke:#f00,fill:#f9f9ff,stroke-width:1px,stroke-dasharray: 5 5
classDef metaCls_rootCls stroke:#f00,stroke-width:5px

• Make Shape working with forwardDims().