[ONNX] Add table with status of supported operators. (#2324)

* Add table with status of supported operators.

* Update comments for unimplemented operators.

* Update comment to tables.

* Update table with Jira tickets.

* Update supported_ops.md

* Fix typo.

src/ngraph/frontend/onnx_import/op/supported_ops.md

+Generally nGraph doesn't support tensors of types:
+
+* `string`,
+* `float16`,
+* `complex64`,
+* `complex128`.
+
+Value in `()` _parenthesis_ indicates that this op was introduced since the specific 
+ONNX Standard opset version. 
+Values seperated by `-` _dash_ indicate the changes were made to that op definition 
+in the ONNX Standard. If there were minor changes they are usually supported by single 
+implementation, otherwise there are multiple versions, each appropriate for specific opset 
+version range.
+For example, with the schema represented below the operator `Abs` is supported in all 
+opset versions starting from `1` to `6` and to the latest opset version.
+
+## Supported Ops:
+
+| Name | Opset supported | Comment |
+|------|----------------------------|---------|
+| Abs | 1-6- |
+| Acos | 7- |
+| Add | 1-7- |
+| And | 1-7- |
+| ArgMax | 1- |
+| ArgMin | 1- |
+| Asin | 7- |
+| Atan | 7 - |
+| AveragePool | 1-7- |
+| BatchNormalization | 1-6-7- | 
+| Ceil | 1-6- | 
+| Clip | 1-6- | 
+| Concat | 1-4- | 
+| Constant | 1- |  
+| Conv | 1- |
+| ConvTranspose | 1- | 
+| Cos | 7- |
+| Div | 1-6-7- | 
+| Dropout | 1-6-7- | Only for inference.
+| Elu | 1-6- |
+| Equal | 1-7 | 
+| Exp | 1-6- | 
+| Flatten | 1-(9) | 
+| Floor | 1-6- | 
+| Gemm | 1-6-7-9 | 
+| GlobalAveragePool | 1- | 
+| GlobalMaxPool | 1- | 
+| Greater | 1-7-9 | 
+| HardSigmoid | 1-6- | 
+| Identity | 1- | 
+| LRN | 1- | 
+| LeakyRelu | 1-6- |  
+| Less | 1-7-9 |
+| Log | 1-6- | 
+| LogSoftmax | 1- | 
+| MatMul | 1-9 | 
+| Max | 1-6-8- | 
+| MaxPool | 1-8- | 
+| Mean | 1-6-8- | 
+| Min | 1-6-8- |
+| Mul | 1-6-7- | 
+| Neg | 1-6- | 
+| Not | 1- | 
+| Or | 1-7- | 
+| PRelu | 1-6-7-9 |
+| Pow | 1-7- | 
+| Reciprocal | 1-6- | 
+| ReduceL1 | 1- | 
+| ReduceL2 | 1- | 
+| ReduceLogSum | 1- | 
+| ReduceLogSumExp | 1- |
+| ReduceMax | 1- |
+| ReduceMean | 1- |
+| ReduceMin | 1- | 
+| ReduceProd | 1- |
+| ReduceSum | 1- | 
+| ReduceSumSquare | 1- |
+| Relu | 1-6- | 
+| Selu | 1-6- | 
+| Shape | 1- | 
+| Sigmoid | 1-6- | 
+| Sin | 7- | 
+| Size | 1- | 
+| Slice | 1- | 
+| Softmax | 1- |
+| Softplus | 1- |
+| Softsign | 1- |
+| Split | 1-2- | 
+| Sqrt | 1-6- | 
+| Squeeze | 1- | 
+| Sub | 1-6-7- | 
+| Sum | 1-6-8- |
+| Tan | 7- |
+| Tanh | 1-6- |
+| Transpose | 1- | 
+| Unsqueeze | 1- |
+| Xor | 1-7- |
+
+## Unsupported Ops:
+
+### Lack of support in nGraph
+| Name | Opset supported | NGCORE | NGONNX | Comment |
+|------|-----------------|--------|--------|---------|
+| Acosh | (9) | 283 | 444 | |
+| Asinh | (9) | 283 | 444 | |
+| Atanh | (9) | 283 | 444 | |
+| Erf | (9) | 284 | 442 | Maybe we may implement this as a simple closed interval integral? :) |
+| Pad | 1-2- | 273 | 416 | Not fully supported. |
+| LSTM | 1-7- | | 430 | Not fully supported. |
+| MaxUnpool | (9) | 286, 289 | 447 | |
+| LpPool | - | 291 | 437 | Further analysis needed, however probably unsupported by nGraph. |
+| Multinomial | - | 199 | 435 | Lack of PRNG in nGraph. |
+| RandomNormal | - | 199 | 434 | Lack of PRNG in nGraph. |
+| RandomNormalLike | - | 199 | 434 | Lack of PRNG in nGraph. |
+| RandomUniform | - | 199 | 434 | Lack of PRNG in nGraph. |
+| RandomUniformLike | - | 199 | 434 | Lack of PRNG in nGraph. |
+| Cast | 1-6- | 290 | 452 | Float16 unsupported. |
+
+### Futher analysis needed
+| Name | Opset supported | NGCORE | NGONNX | Comment |
+|------|-----------------|--------|--------|---------|
+| GRU | - | | 325, 177 | Should be possible to implement. Look at `LSTM` |
+| RNN | - | | 323, 287 | Should be similar to `LSTM`. |
+| If | - | | 432 | At this moment probably impossible. |
+| IsNaN | (9) | | 440 | Hacky way is to generate constant nodes with representations of NaN and compare with them. |
+| Loop | - | | 432 | Static loops with some preconditions may be possible, however no idea how to pass graph (proto?) as a _body_ attribute. (what about graph contains `Loop`?) |
+| OneHot | (9) | | 453 | Furhter analysis needed. Unclear nGraph doc of `OneHot` op. |
+| Scan | - |  | 433 | Further analysis needed. - determine whether it is possible to import graph passed by op attribute. |
+
+### Dynamic operators
+| Name | Opset supported | NGCORE | NGONNX | Comment |
+|------|-----------------|--------|--------|---------|
+| Compress | (9) | 285 | 438. | Dynamically selected indices |
+| ConstantOfShape | (9) | 286 | 445 | Dynamic shape input. |
+| Expand | - | NGRAPH-3289 | 367 | Dynamic op. |
+| Gather | - | NGRAPH-3291 | 369, | Dynamic op.  |
+| Tile | - | NGRAPH-3292 | 368 | Dynamic op. |
+| Upsample | (7) | 287 | 441 | Dynamic op. |
+| MaxRoiPool | - | 288 | 437 | Dynamic op. Beside just use _slice/op/concat_ pattern. |
+| Reshape | 1-5- | NGRAPH-3290 | 357 | Lack of support for dynamic shape input. Only as a Constant or as an Initializer. |
+| Scatter | (9) | 289 | 446 | Dynamic indices input. |
+
+### Able to implement or WIP
+| Name | Opset supported | NGCORE | NGONNX | Comment |
+|------|-----------------|--------|--------|---------|
+| Cast | 1-6- | | 427 | Errors while casting to bool |
+| EyeLike | (9) | | 439 | Make constant node. |
+| GlobalLpPool | - | | 437 | Probably use _slice/op/concat_ pattern. |
+| Hardmax | - | | 431 | Use make constant and Argmax. See `test_ops_unary.py::test_hardmax()` |
+| LpNormalization | - | | 436 | Just an equation. Only Lp{1,2} need to be supported. |
+| InstanceNormalization | - | | 436 | Just an equation. For per channel computation may _slice/op/concat_ pattern need to be used. |
+| Shrink | (9) | | 449 | Just an easy equation. |
+| TopK | - | | 327. | Use nGraph `Topk`. |
+| Cosh | (9) | | 448 | Use nGraph `Cosh`. |
+| Sign | (9) | | 448 | Use nGraph `Sign`. |
+| Sinh | (9) | | 448 | Use nGraph `Sinh`. |
+| Where | (9) | | 448 |  Use nGraph `Select`. |