Add issue templates.

.github/ISSUE_TEMPLATE/blank_issue.yml

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+name: Blank Issue
+description: Submit any other kind of issue.
+labels: [Blank Issue]
+body:
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+      description: Please describe the issue here.
+      placeholder: Description
+    validations:
+      required: false

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: BUG Report
+description: Report a BUG of Tensorflow.NET.
+title: "[BUG]: "
+labels: [bug-report]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        To help us fix your problem more quickly, please check the following steps at first.
+        - [ ] I have read the related documents.
+        - [ ] I have searched the keywords in the issues.
+  - type: textarea
+    id: background
+    attributes:
+      label: Description
+      description: Please share a clear description of the problem.
+      placeholder: Description
+    validations:
+      required: true
+  - type: textarea
+    id: repro-steps
+    attributes:
+      label: Reproduction Steps
+      description: |
+        Please describe how to reproduce the problem here. A minimal example code is the best.
+      placeholder: Reproduction Steps
+    validations:
+      required: true
+  - type: textarea
+    id: configuration
+    attributes:
+      label: Environment & Configuration
+      description: |
+        Please provide the information of your environment and configuration.
+      placeholder: Environment & Configuration
+      value: |
+        Operating system: 
+        .NET runtime version: 
+        LLamaSharp version:
+        CUDA version (if you are using cuda backend): 
+        CPU & GPU device: 
+    validations:
+      required: true
+  - type: textarea
+    id: known-workarounds
+    attributes:
+      label: Known Workarounds
+      description: |
+        Please provide a description of the known workarounds, if any.
+      placeholder: Known Workarounds
+    validations:
+      required: false

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: Feature Request
+description: Request/Propose a new feature in LLamaSharp.
+title: "[Feature]: "
+labels: [feature-request]
+body:
+  - type: markdown
+    attributes:
+      value: |
+        Feature proposal/request is always welcomed!
+  - type: textarea
+    id: background
+    attributes:
+      label: Background & Description
+      description: Please describe the purpose and value of the new feature here.
+      placeholder: Background & Description
+    validations:
+      required: true
+  - type: textarea
+    id: api-proposal
+    attributes:
+      label: API & Usage
+      description: |
+        Please tell us the new APIs related to the feature, if any. Please describe when and how it is used.
+      placeholder: API & Usage
+    validations:
+      required: false
+  - type: textarea
+    id: implementation
+    attributes:
+      label: How to implement
+      description: |
+        Please describe how you think the feature should be implemented. It's okay leave it blank.
+      placeholder: How to implement
+    validations:
+      required: false